feat(renderer): wire remaining stubbed commands and rebuild

The v5.0.0 React UI shipped with a working surface but had a number of
features left as 'wired in a later phase' or broken by missing IPC
plumbing. This commit completes the wiring end-to-end and proves the
behaviour with the run-desktop driver.

Toolbar format buttons (Toolbar.tsx)
  - Bold, Italic, Unordered/Ordered list, Code, Link now dispatch real
    commands instead of being permanently disabled.

New module: src/renderer/lib/editor-commands.ts
  - Module-level singleton holding the active CodeMirror EditorView.
  - toggleBold/Italic/Code/CodeBlock, toggleUnorderedList/OrderedList,
    insertLink, setHeadingLevel, scrollToLine, undo, redo, insertSnippet.
  - CodeMirrorEditor mounts the view via setActiveView on mount and
    nulls it on unmount, so commands land on the focused buffer.

register-menu-commands.ts
  - file.confirmClose: prompts before closing a dirty tab via the
    confirm modal; otherwise closes silently.
  - app.quit: prompts before quitting if any buffer is dirty; uses
    the new ipc.app.quit channel.
  - short-cuts.show: shows the keyboard shortcut list in a confirm dialog.
  - editor.bold/italic/code/list.*/link/undo/redo/heading.*: drive the
    active editor.
  - find.toggle: dispatches mc:find-toggle for any listening component.
  - view.sidebarPanel/bottomPanel: navigate between sidebar sections
    via the new data-testid buttons; bottom panel toggles the REPL.
  - font.size: increase/decrease/reset on editorFontSize (10..28).
  - theme.loadCustomCss/clearCustomCss: pick and clear the custom CSS
    path on the settings store.
  - template.load: inserts a bundled markdown skeleton at the cursor.
  - print.preview/previewStyled: emit mc:print and mc:print-preview.
  - file.clearRecent: clears open tabs.
  - file.new: creates an untitled buffer and tab.
  - editor.gotoHeading: scrolls to a given line (used by Outline and
    Breadcrumb).
  - view.toggleSidebar: already wired.
  - file.opened: already wired (prior fix).

Sidebar Outline + Breadcrumb
  - Both now click through to 'editor.gotoHeading' with the line
    number extracted from the active buffer.
  - Sidebar exposes data-testid for the 'view.sidebarPanel' menu action.

editor-commands sets the active view; scroll metrics flow through to
Minimap which now reads scrollRatio/visibleRatio from the editor.

Export dialogs (PDF/DOCX/HTML)
  - Replaced the broken ipc.export.{pdf,docx,html,batch} calls (those
    channels were not implemented in main and only CHANNEL_MISSING
    was returned) with renderer-side pipelines.
  - PDF: generateHtml() with @page CSS for size + margins, then
    ipc.print to the main process for native print-to-PDF.
  - DOCX: generateDocx() from the existing lib, then ipc.file.writeBuffer.
  - HTML: generateHtml() then ipc.file.writeBuffer.
  - All three dialogs use ipc.app.showSaveDialog for output path
    (new IPC handler) and ipc.file.writeBuffer for the binary write
    (new preload channel).
  - PrintPreview now uses MarkdownRenderer instead of a raw <pre>.

New settings fields
  - editorFontSize: 10..28 px (drives CodeMirror theme font-size)
  - customCssPath: string|null (Theme menu wires 'load-custom-css' and
    'clear-custom-css' to it).

New IPC channels
  - 'app:quit', 'app:open-external', 'app:show-save-dialog',
    'write-buffer' (already existed; now exposed in preload).

Tests
  - Updated Toolbar test: format buttons are no longer disabled and
    dispatch their command ids.
  - Updated Export*D*Dialog tests to mock the new ipc surface
    (ipc.print, ipc.app.showSaveDialog, ipc.file.writeBuffer) and
    assert the new flow.
  - Updated PrintPreview test to use ipc.print.doPrint.
  - Updated phase8-toasts integration: PDF flow now goes through
    ipc.print and asserts 'Sent <title> to printer'.
  - register-menu-commands test: re-register 'template.load' AFTER
    Harness renders so the captured-args handler is the live one.

Verification
  - npm run build:renderer: success (1.6s)
  - npx vitest run: 308 passed (up from 305; 3 new + unchanged)
  - npx jest: 189 passed (unchanged)
  - .claude/skills/run-desktop/verify-features.mjs: drove the live app
    via Playwright, exercised every wired feature end-to-end, captured
    11 screenshots. All verified working (editor + preview render the
    file content, toolbar buttons dispatch, dialogs open, theme
    toggles, outline shows headings, italic button works in editor).

tests/integration/phase8-toasts-smoke.test.tsx

@@ -16,15 +16,18 @@ vi.mock('@/lib/ipc', () => ({
     app: {
       getVersion: vi.fn().mockResolvedValue({ ok: true, data: '5.0.0' }),
       openExternal: vi.fn().mockResolvedValue({ ok: true }),
+      showSaveDialog: vi.fn().mockResolvedValue({ ok: true, data: '/test.pdf' }),
     },
     file: {
       read: vi.fn(),
       write: vi.fn(),
+      writeBuffer: vi.fn().mockResolvedValue({ ok: true }),
       list: vi.fn(),
       pickFolder: vi.fn(),
       pickFile: vi.fn(),
       onChange: vi.fn(),
     },
+    print: vi.fn().mockResolvedValue({ ok: true }),
     menu: {
       on: vi.fn(() => () => {}),
     },
@@ -90,7 +93,7 @@ describe('Phase 8 toasts integration', () => {
   });
 
   it('exporting a file calls toast.success on success', async () => {
-    (ipc.export.pdf as any).mockResolvedValue({ ok: true, data: { outputPath: '/test.pdf', bytes: 100, durationMs: 50 } });
+    (ipc.print as any).mockResolvedValue({ ok: true });
     registerMenuCommands();
     render(<App />);
 
@@ -102,8 +105,10 @@ describe('Phase 8 toasts integration', () => {
     const exportBtn = await screen.findByRole('button', { name: /^export$/i });
     await userEvent.click(exportBtn);
 
+    // The PDF flow hands the rendered HTML to the main process for print.
     await waitFor(() => {
-      expect(toast.success).toHaveBeenCalledWith(expect.stringContaining('Exported test.md'));
+      expect(ipc.print).toHaveBeenCalledTimes(1);
     });
+    expect(toast.success).toHaveBeenCalledWith(expect.stringContaining('Sent test.md'));
   });
 });