feat(agent): resizable, undockable assistant panel (#1467)

* feat(agent): resizable, undockable assistant panel

User report: the assistant chat sheet sometimes covers the page content
the user is asking about, with no way to resize or move it.

- Docked mode is now drag-resizable from its left edge (380-800px,
  clamped so the page keeps a 480px readable column) and the page
  reflows beside it via the existing --agent-dock-w reservation.
- Expanded (focus) mode reserves page margin like the compact dock
  instead of overlaying up to 1100px of the page.
- New undock toggle turns the sheet into a floating window that can be
  dragged by its header and resized from edges/corners, clamped so the
  header always stays reachable. Desktop only; mobile keeps the
  full-screen sheet.
- Geometry (mode, dock width, float rect) persists per user in
  user_preferences.ui_state.agent_panel, server-seeded to avoid a
  first-paint jump; the ui-state API schema gains a strict agent_panel
  key with nested merge.
- Pure clamp/resize math lives in lib/agent-panel/geometry with unit
  tests; drag frames write styles imperatively and commit one
  preference update on release.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(agent): address review findings on panel drag, a11y, and persistence

CodeRabbit round 1, all six findings fixed:

- Bind drag listeners to window (plus lostpointercapture) so a failed
  pointer capture or mid-drag unmount can never leave the transition
  suppression and data-agent-resizing stuck for the session.
- Keyboard resize now steps from the visible width (expandedW in focus
  mode) instead of jumping to the persisted dock width.
- The width handle exposes window-splitter semantics: aria-valuenow,
  aria-valuemin, aria-valuemax.
- --nav-w is read reactively via a MutationObserver on #dash-shell
  instead of computed-style reads in the render body and per drag frame.
- The ui-state POST in updatePanelPrefs gets a 300ms trailing debounce
  (state stays immediate) so key auto-repeat cannot produce one
  read-merge-write per repeat; pending write flushes on unmount.
- globals.css keeps one :root token block; the agent-resizing rule moved
  below it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(agent): filter drag events by pointer id, clear fired debounce timer

CodeRabbit round 2, both findings fixed:

- Window-level drag listeners now ignore events from pointers other than
  the initiating one, so a second touch or pen cannot move the panel or
  end the first pointer's drag.
- The persist debounce timer ref is nulled when the timer fires, so the
  unmount flush only writes genuinely pending values instead of
  replaying an already-persisted (possibly stale) geometry.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

app/globals.css

@@ -103,6 +103,14 @@
   --duration-slow: 500ms;
 }
 
+/* While the assistant panel's width handle is being dragged, the page margin
+   must track the cursor 1:1: the page panel's 300ms margin transition would
+   trail the drag and leave a gap. AgentSheet sets the attribute for the
+   duration of the drag only. */
+:root[data-agent-resizing] #main-content {
+  transition: none !important;
+}
+
 .dark {
   color-scheme: dark;