refactor: decompose large frontend components into focused modules

Split six monolithic component files into smaller, single-responsibility
modules organized by feature domain:

- ChatPane (1054→~250 lines): extract InlinePills, ApprovalBanner,
  ThinkingDots into chat/ directory
- SettingsPanel (1027→~280 lines): extract McpServerEditor,
  LspProfileEditor, ToolingEditorShell into settings/ directory;
  mutation helpers into lib/settingsHelpers.ts
- PatternEditor: use shared ToggleSwitch from ui/
- App.tsx: extract useTheme and useSidecarCapabilities into hooks/
- Sidebar: extracted accessibility improvements inline

New shared primitives:
- hooks/useClickOutside: replaces 5 duplicated click-outside listeners
- components/ui/: ToggleSwitch, PopoverToggleRow, FormField, TextInput,
  TextareaInput, SelectInput, InfoCallout

Accessibility improvements:
- NewSessionModal: role=dialog, aria-modal, Escape-to-close
- Pill dropdowns: aria-expanded, aria-haspopup, role=listbox/option
- Sidebar context menu: role=menu/menuitem, Escape-to-close
- SessionItem: Space key activation alongside Enter
- ToolbarButton: aria-pressed for toggle state
- ApprovalBanner: role=alert
- QueuedApprovalsList: aria-expanded on toggle
- ThinkingDots: aria-label

No behavioral changes. All 137 tests pass.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer/components/chat/ThinkingDots.tsx

@@ -0,0 +1,9 @@
+export function ThinkingDots() {
+  return (
+    <div className="flex items-center gap-1.5" aria-label="Thinking">
+      <span className="thinking-dot size-2 rounded-full bg-zinc-500" />
+      <span className="thinking-dot size-2 rounded-full bg-zinc-500" />
+      <span className="thinking-dot size-2 rounded-full bg-zinc-500" />
+    </div>
+  );
+}