Land the accepted navigation model: one app bar for every page

[mtaanquist]

Ports the accepted navigation model from design-pass session 2. Every page now
wears one bar; the three bars it replaces are gone. Behaviour is unchanged
except where the model changes it.

What migrated, per screen

Screen	Now
Editor (Write, Plan, Notes, Review)	.appbar with the full path; the story name is a menu, "Go to the story" first. Save pill stays in the tools cluster
Universe Plan and Notes	.appbar; the universe crumb has one destination, its Plan. Four modes with the mode-note line
Library	.appbar with Library as current text; each universe row carries "Insights" and "Settings" text buttons in place of the gear
Story settings	.appbar with the full path; sidebar swapped to .side-nav; the back link is gone
Universe settings	.appbar with the full path; .side-nav plus a "Back to the universe" group (Plan, Insights)
Insights	.appbar with Library / Universe / Insights; .side-nav over its existing in-page anchors
Account, admin	.appbar with Library / Account and Library / Site admin. Their icon-led .admin-nav sidebars stay as they are: not a straight class swap
Print preview	.appbar with the full path; Print and Page setup are page-header actions; the one-off print bar is deleted; the paper wears data-theme="light" so its white and its ink are tokens
Guest review	.appbar.guest with the "Review only" pill, theme and help tools, and "Reviewing as ..."
Help	.appbar plus the article list in the left slot and a "Back to your work" group. The bare pages and the modal are both gone
Reading pages (/@handle, story reader)	.appbar.reader with theme support. Prose presentation untouched; full reader retheming is session 3

Deleted

Components: HelpModal.svelte, TopBar.svelte, PageTopBar.svelte,
PaletteButton.svelte (folded into the bar's .tool-search).

CSS, each grepped for references first: .topbar and .page-shell .topbar,
.crumbs/.crumbs .sep, .topbar-right, .brand-mark, .back-link, the
.breadcrumb family, .divider, .save-status, the .review-guest-bar
family, .universe-edit. .app.focus-mode .topbar became
.app.focus-mode .appbar.

Ratified decisions

1. Place-crumbs are menus. Clicking the universe or story name opens its
   menu; "Go to the universe" / "Go to the story" is the first item, ticked
   when you are already there. No split-click, no gear anywhere in the chrome.
   Menus are real popovers composing .popover/.menu-item, with the repo's
   dismiss action, aria-haspopup/aria-expanded, arrow keys and Escape.
2. The help modal is deleted. The ? navigates to the help pages, lights
   up via aria-current while there, and returns to the previous location
   when pressed again.

Visible differences to expect, all intentional

• The story title in the path no longer opens settings. It opens a menu that
  names every destination, settings included.
• There is no gear anywhere in the chrome, and no icon-only destination.
• The universe crumb always goes to the same place.
• Help never opens over the top of your work.
• The print page and the reading pages gained 52px of chrome at the top.
• Guests gained a theme control and a help link, and lost nothing.
• The mode strip is the same width on every screen that has one.

Chrome layer

chrome.css landed as src/lib/styles/chrome.css, imported last in
+layout.svelte after primitives.css. Two fixes while landing it, as
briefed: the header comment described an abandoned iteration ("settings is
never a crumb, it is the gear at the end of the path") and now describes the
final model; .tool-search .kbd targeted a class the primitives
consolidation deleted and now targets the kbd element.

Theme tool

Reconciled with the existing theme.ts rather than added beside it.
flipTheme (light/dark, resolving through the saved system mappings) is
replaced by cycleTheme, which walks the three concrete palettes
dark -> light -> warm. ThemeToggle.svelte became the bar's theme tool, and
UserMenu's theme item now calls the same cycleTheme so there is one
cycle in the product, not two. Signed-in viewers persist through the existing
appearance API; guests and readers keep it in localStorage.

Scope fences honoured

• Right pane untouched. The tab strip is still
  Reference/History/Session/Assistant. The nav-write mock's "Notes 2" tab was
  ignored as filler.
• Universe overview restructure deferred. The universe Plan page keeps its
  existing PlanSidebar entity list. Only the app bar, the four-mode strip
  with its note, and a small "Across the universe / Insights" group at the
  foot of the sidebar landed. The mock's side-nav of
  Overview/Characters/Places/... and its new centre sections are not built.
• Settings content untouched. No universe Colour field (the mock invents
  one; the schema has no such column), no section renames, no new sections.
  Only chrome, paths, and the sidebar skin changed.
• No schema changes.

Two things worth a decision

• The reading-page menu item appears only when the story has a current,
  non-removed edition, so the menu never offers a link that 404s. That needed
  a readingPageRef helper in server/publish.ts and a reading field on
  the six story-scoped page loads. When there is no edition the item is
  omitted entirely.
• /docs is public now (PUBLIC_PATHS plus a /docs/ prefix). The guest
  and reader shells keep the help tool per the shells table, and neither has
  an account, so help had to stop redirecting to /login.
• The universe mode-note wording is the mock's verbatim: "Write and Review
  need a story open. Both take you to the story list below." In our layout the
  story board is the centre column, to the right of the strip, not below it.
  Kept as briefed, flagged here: say the word and it becomes "Both take you to
  this universe's story list".

Docs and kit

• design-system.md: new Chrome section (app bar and the three shells, path
  and crumb-menu rules, side-nav, tool-search, mode-strip constants, and the
  rule that anything reachable only from the palette is a bug), shells table
  updated, and the stale TopBar/PageTopBar/HelpModal references cleared
  from the modal section, the new-page checklist and the component index.
• design.md: the navigation paragraph, the universe four-mode behaviour, and
  the focus-mode sentence now match what ships.
• Help articles: editor.md (the gear/breadcrumb sentence is now the story
  menu), getting-started.md (the ? is a page you can go back from),
  account.md.
• New kit card scratch/design-kit/appbar.html with the three shells stacked
  and the crumb menu open, added to the README card list.

Verified locally

• npm run lint clean, npm run check clean (0 errors, 0 warnings).
• npm run test:unit: 1077 passed, 1 skipped, 132 files.
• e2e: 49 passed, 3 failed. All three fail at the same "Prepare ... (.zip)"
  export step (core-flow, import-story, universe-settings): this
  checkout has no .env and so no ASSET_S3_BUCKET, which is the documented
  local baseline. Every chrome assertion passes, including the rewritten ones.
• Specs updated to the new chrome, keeping the assertions at least as strong
  and preferring role and label selectors: core-flow (crumb menu contents
  and its ticked current item, .appbar in focus mode, settings reached
  through the story menu, the help tool as a place), docs (path, ticked
  article, plus a new test that the ? navigates and returns), review
  (guest shell tools, the four-mode strip with three aria-disabled and the
  note), account (the three-palette cycle; Theme disambiguated from the
  bar's theme tool).
• Render-verified the new kit card headless in all three themes and read the
  screenshots, plus the library, docs article, print preview, story settings,
  universe Plan and the open story crumb menu in the running app.

Not verified locally

The export-dependent paths (they need MinIO), and everything in core-flow
after its export step, which the local suite cannot reach for the same reason.
CI is the backstop and earned it: it caught one stale selector in that
unreachable stretch, page.locator('.brand'), left over from the deleted
bars. Fixed in the second commit to select the brand by its accessible name.
All three CI checks pass now: Lint/check/unit/build, End-to-end, Docker stack
boots.