feat(dashboard): UI polish pass — resizable modals, themed scrollbars, settings overhaul

Modal resize + size persistence
- Extract `useModalResizePersist` hook (ResizeObserver + localStorage),
  apply to Files, Git Manager, GitHub Import, Workflow Steps, Automations
  (ScheduledTasks), and Settings modals. Each gets `resize: both`, sane
  min/max constraints, and a unique storage key.
- Bump default heights so the modals feel less cramped (Git: 92vh,
  Workflow/Automation: 80vh, Settings: 80vh / 1100px).

Scrollbar theme
- Add a global `*::-webkit-scrollbar*` + `scrollbar-color` rule in
  styles.css so chat, document, system stats, file browser, usage
  indicator, etc. inherit the theme. Existing per-component overrides
  (.board, .column-body, .settings-sidebar, planning modal) still win.

Document view
- Collapse "Show hidden" toggle and search input onto the same row as
  the Project Files / Task Documents segmented control. Stack again
  below 768px.

Mailbox / Todos
- Match Todos header treatment to the Mailbox header (typography,
  padding, border).
- Add top spacing above Mailbox Inbox/Outbox/Agents tab bar so the
  vertical gaps balance.

Settings
- Wider, resizable, persisted Settings modal.
- Project Models description and Authentication panel get proper
  horizontal padding.
- Reorder project sidebar so "General" is first.
- Plugins page: clean margins, integrate refresh button, exclude
  bundled runtimes from the "Installed Plugins" list (they were
  appearing twice — once erroring, once in their own section).
- New "Updates" panel with auto-check toggle + "Check now" button
  (frequency control noted as needing a backend schema field).

Background sessions
- Fix stale "AI N" / planning-icon badge: `handleDeleted` in
  `useBackgroundSessions` now writes a tombstone, advances the
  timestamp guard, and broadcasts completion so the cross-tab
  sync store stops resurrecting the deleted session on the next
  merge tick. Added regression test.
- Re-fetch list on SSE reconnect so terminal events fired during a
  network blip don't get permanently lost.

System stats
- Refresh button uses correct single class (was getting both `btn`
  and `btn-icon`, which conflicted on padding/border).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/dashboard/app/styles.css

@@ -7,6 +7,44 @@
   padding: 0;
 }
 
+/* === Global Scrollbar Theme ===
+   Low-specificity wildcard rules so all scrollable surfaces pick up the
+   dark theme by default. Per-component overrides (e.g. .board, .column-body,
+   .settings-sidebar, .settings-content, .planning-modal *) remain unaffected
+   because their class-level selectors have higher specificity than * . */
+* {
+  scrollbar-color: var(--border) transparent;
+  scrollbar-width: thin;
+}
+
+*::-webkit-scrollbar {
+  width: 8px;
+  height: 8px;
+}
+
+*::-webkit-scrollbar-track {
+  background: transparent;
+}
+
+*::-webkit-scrollbar-thumb {
+  background: var(--border);
+  border-radius: var(--radius-sm);
+}
+
+*::-webkit-scrollbar-thumb:hover {
+  background: var(--text-muted);
+}
+
+*::-webkit-scrollbar-corner {
+  background: transparent;
+}
+
+/* Firefox app-wide scrollbar via html element */
+html {
+  scrollbar-color: var(--border) transparent;
+  scrollbar-width: thin;
+}
+
 /* === Utility Classes === */
 .visually-hidden {
   position: absolute;