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/ui/ToggleSwitch.tsx

@@ -0,0 +1,24 @@
+export interface ToggleSwitchProps {
+  enabled: boolean;
+  size?: 'sm' | 'md';
+}
+
+export function ToggleSwitch({ enabled, size = 'md' }: ToggleSwitchProps) {
+  const trackSize = size === 'sm' ? 'h-[14px] w-[24px]' : 'h-[18px] w-[32px]';
+  const thumbSize = size === 'sm' ? 'size-[10px]' : 'size-[14px]';
+  const translateOn = size === 'sm' ? 'translate-x-[12px]' : 'translate-x-[16px]';
+
+  return (
+    <span
+      className={`relative inline-flex ${trackSize} shrink-0 items-center rounded-full transition-colors ${
+        enabled ? 'bg-indigo-500' : 'bg-zinc-700'
+      }`}
+    >
+      <span
+        className={`inline-block ${thumbSize} rounded-full bg-white shadow-sm transition-transform ${
+          enabled ? translateOn : 'translate-x-[2px]'
+        }`}
+      />
+    </span>
+  );
+}