feat(mobile): chat polish — floating composer + iOS keyboard handling + bottom-sheet drag-close + file-chip overflow

Multi-pass mobile chat polish landed across this session.

- MessageInput is now a floating glass-bubble (`position: absolute`) on both desktop and mobile — last messages scroll *behind* the translucent bubble. MobileChatScreen wraps MessageList + MessageInput in a `relative` parent so absolute positioning resolves. Removed the prior in-flow mobile branch that clipped message-list bottom against an invisible barrier.
- iOS PWA keyboard handling: new `useVisualViewportInset` hook subscribes to `visualViewport.resize/scroll` AND polls `vv.height` for ~600ms after focusin (iOS PWA standalone often fails to dispatch resize for keyboard transitions). MobileShell sizes its container to `vv.height` when keyboard is open — `bottom: 0` on the composer naturally lands flush with the keyboard top, regardless of how reliably resize events fire. Composer uses 6px gap above home indicator (keyboard closed) and 0px gap above keyboard (keyboard open). Added `interactive-widget=resizes-content` viewport meta as the cleaner native equivalent for Chrome/Android.
- MessageList bottom padding is dynamic via `--composer-clearance` CSS variable. MessageInput writes `composerHeight + bottomOffset + 12px` to its parent via ResizeObserver — re-fires on textarea autosize, reply banner, attachment tile growth, parent resize. Last message always has 12px breathing room above the bubble regardless of composer state.
- AttachmentRenderer generic file chip: `max-w-full sm:max-w-[400px]` on outer + `min-w-0` + `flex-shrink-0` on icon + `flex-1` on text + `flex-wrap` on badge row. Long filenames now ellipsize cleanly on narrow viewports instead of pushing the chip off-screen.
- New `useDragToClose` hook: shared bottom-sheet drag-down-to-dismiss gesture. Spread on handle/header only (body scrolling unaffected). 6px deadzone, 100px or 0.5px/ms velocity threshold, 200ms `cubic-bezier(0.22, 1, 0.36, 1)` close-out animation, rAF-staged transform for a stable from-value. `hasInteracted` latch prevents the open keyframe from re-firing mid-close (the bounce-up-then-vanish bug). Wired into InputPopover (emoji/GIF), MobileVoiceJoinSheet, MobileFolderSheet.

Specs: docs/systems/mobile-ui.md (Floating Composer + Drag-to-Close sections), docs/systems/message-list.md (--composer-clearance), docs/systems/design-system.md (glass-bubble row references).

docs/systems/message-list.md

@@ -11,6 +11,12 @@ The chat message list (`packages/web/src/components/chat/MessageList.tsx`) is re
 | `packages/web/src/components/chat/embeds/*.tsx` | Embed renderers — must obey the dimension reservation contract below. |
 | `packages/web/src/components/chat/AttachmentRenderer.tsx` | Reference for the dimension reservation pattern (`AttachmentRenderer.tsx:81-100`). |
 
+## Bottom clearance — `--composer-clearance`
+
+The MessageList content's `paddingBottom` is dynamic — it reads the CSS variable `var(--composer-clearance, 80px)` written to the chat region's wrapper element by `MessageInput` via a `ResizeObserver`. The variable's value is `composer.height + composer.bottom-offset + 12 px` so the last message always lands 12 px above the composer's top edge regardless of (a) composer height (reply banner, staged-attachment tile row, multi-line autosize), (b) the composer's own `bottom` style (desktop 12 px / mobile-keyboard-closed `env(safe-area-inset-bottom) + 6` / mobile-keyboard-open 0).
+
+The 80 px fallback is sized for the desktop case during the brief mount window before the first ResizeObserver tick — it matches the previous static `pb-20`. See `docs/systems/mobile-ui.md` "Floating Composer" for the full rationale; the short version is that any static value is wrong on iPhone (composer height + safe-area + 6 already exceeds 80 px) and wrong when the composer grows (replies, attachments).
+
 ## ContainerRef invariant
 
 `containerRef.current` (and `contentRef.current`) MUST be non-null for the entire lifetime of any chat view where messages may arrive or scroll-affecting effects can run. Every effect in this file (initial snap A, ResizeObserver B, capture-phase load handler C, scrollend final-pin D, jump-to-message) reads these refs and bails on a null guard. Each of those effects is keyed on `[messages.length, channelId]` or `[hasMessages, channelId]`, and the inbound transition (`0 → N` / `false → true`) is the *only* re-fire signal during a channel-load lifecycle. If a ref is null at the moment that signal fires, the effect bails — and no later dep change will retry it, leaving the channel permanently broken (initial scroll never lands at bottom, ResizeObserver never observes, scrollend never registers, saved-anchor restore never happens).