Fix #17: per-sample description missing on first nav + scrollbar/schema fixes

[TimPurdum]

Follow-up fixes for #17 addressing Tim's testing feedback and the two unresolved Copilot review comments.

Tim's feedback

"About this page" never shows on first navigation from the navbar, only after a page refresh.
The Schema.org LD-JSON is too static for the site, each page should have somewhat different content, especially the description and url.

Root cause for both: the MainLayout.OnLocationChanged handler cleared LayoutService.CurrentPage on every navigation, expecting the next SamplePage.OnInitialized to re-register. But on first nav from the navbar, the layout's render after the clear could land before the new SamplePage had run its OnInitialized, so the description / SoftwareSourceCode / per-page meta tags rendered against CurrentPage == null. That's also why the JSON-LD looked "static" — the per-page description / SoftwareSourceCode entry was missing on first paint.

Fix: key LayoutService.CurrentPage to the URI it was registered for. SamplePage.OnInitialized now passes NavigationManager.Uri when calling SetCurrentPage. MainLayout reads through a new EffectiveCurrentPage getter that only returns the registered page when its URI matches NavigationManager.Uri. No timing dependency, and stale references after navigating to a non-SamplePage route (Home/NotFound) are silently ignored — same guarantee the explicit ClearCurrentPage was trying to provide, without the race.

While there, enrich the per-page JSON-LD with keywords derived from the page name plus mainEntity / mainEntityOfPage cross-references between WebPage and SoftwareSourceCode, so each sample page emits visibly distinct schema content beyond just url/name.

Unresolved Copilot comments addressed

• Nested scrolling on narrow layouts (MainLayout.razor.css:63): article.content had a fixed height: calc(100vh - 10rem) + overflow-y: auto at all widths, and on <1075px so did the parent <main> — double scrollbars. Moved both into the >=1075px media query, where <main> switches to overflow:hidden and inner article scrolling is what we want.
• Stale CurrentPage after navigating to a non-SamplePage route: subsumed by the URI-key approach above (the thread was marked resolved by the original ClearCurrentPage change, but the new approach handles both this case and Tim's first-nav case with a single mechanism).

Other Copilot comments were already addressed in the existing PR commits (Dispose() no longer disposes the injected HttpClient; summary::marker cleared for cross-browser parity; MetaTitle only appends "— GeoBlazor Sample" when on a sample page; SlugToTitle takes the last /-segment; CanonicalPageUrl strips query/fragment).

Files changed

• LayoutService.cs — add PageUri; SetCurrentPage(page, uri); drop ClearCurrentPage (no longer needed)
• SamplePage.cs — inject NavigationManager; pass Uri when registering
• MainLayout.razor — EffectiveCurrentPage getter; use it in render, meta tags, and JSON-LD; OnLocationChanged now just calls StateHasChanged; enrich JSON-LD per-page
• MainLayout.razor.css — move article fixed-height + overflow into >=1075px media query

Test plan

• On first-time navigation from navbar (cold load to Home, then click a sample link), "About this sample" details block renders without refresh.
• Per-page <meta name="description">, og:url, and JSON-LD WebPage/SoftwareSourceCode reflect the new sample on each navigation.
• Navigate from a sample to Home — description disappears, no stale per-page meta.
• On <1075px viewport, no nested/double scrollbars on sample pages with long content.
• On >=1075px, article still scrolls independently inside main as before.

Targets feature/schema-org-jsonld so it stacks into #17.

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