[Mobile] The group-chat surfaces are unadapted below 640 px: message rows, composer, attachments and approval cards #1147

Owner: Dev3

The in-repo half of the mobile decision in #647 (PWA first). #647 keeps the half this repository cannot do — the manifest and the icon pair live in the DSH web-frontend package — and stays needs-human; this issue is the part that is ours.

Why this is a real gap and not polish

plugins/social/client.js has exactly one @media block, line :258 (.fs-hold Approve/Reject at 40px). Every other surface this plugin draws — group message rows, the composer and its send button, attachment lines, approval cards — is laid out on desktop widths and simply shrinks below 640 px. core/src/client/mobile.js (MOBILE_MAX_WIDTH = 640, :8) covers core's own surfaces; it does not reach the plugin's.

Scope (one plugin, three paths)

At 375 px and at 640 px, without horizontal scrolling of the message column:

1. Group message rows — a long unbroken run (a URL, a code block, a Bot's tool output) wraps inside the bubble; the sender label and timestamp do not collide with the text.
2. Composer and send — the input keeps its usable width with the send button present, both touch targets at least 40 px, and the composer stays above the safe-area inset rather than behind it.
3. Attachments and approval cards — attachment lines truncate with the file name visible, the card's action row wraps instead of overflowing, and each action is a 40 px+ target (this is the one path :258 already does for held replies — keep it and extend the shape, do not restyle it away).

Acceptance

1. Each of the three paths gets at least one assertion, not only new CSS. A CSS string assertion in plugins/social/test/ is acceptable if that is what the surface allows — say so in the PR and point at the test. Deleting the new rule must turn exactly that assertion red.
2. plugins/social stays green with 0 skipped, and core/ is not touched.
3. No change to what any of these surfaces says or when it appears — layout only.
4. If a rule cannot be verified in this repo (see the safe-area note below), it does not go in.

Hard constraints

• Do not touch the shell or the DSH package. rel=manifest, display, the icons and viewport-fit=cover are DSH's (AGENTS.md hard rule 1). Overriding the shell's <link rel=manifest> from a plugin is the same rule from the other side.
• env(safe-area-inset-bottom) is 0 on this stack today. The shipped index.html has width=device-width, initial-scale=1 and no viewport-fit=cover, so core/src/client/mobile.js:19's inset is inert and any new inset-based rule for this plugin would be unverifiable. Write the composer rule so it is correct at inset 0 now and does not depend on the inset; the shell half is tracked in [Mobile] Android 客户端支持 #647.
• Reviewer: in the PR must not be Dev3.