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/components/PluginManager.css

@@ -5,6 +5,8 @@
   display: flex;
   flex-direction: column;
   gap: var(--space-lg);
+  padding-inline: var(--space-xl);
+  padding-block: var(--space-md);
 }
 
 .plugin-manager-header {
@@ -13,6 +15,14 @@
   justify-content: space-between;
   padding-bottom: var(--space-sm);
   border-bottom: var(--btn-border-width) solid var(--border);
+  gap: var(--space-sm);
+}
+
+.plugin-manager-header-title {
+  font-size: 13px;
+  font-weight: 600;
+  color: var(--text);
+  flex: 1;
 }
 
 .plugin-manager-actions {