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).

src/renderer/lib/pdf-export.ts

@@ -0,0 +1,50 @@
+/**
+ * Renderer-side Markdown → PDF export.
+ *
+ * Opens the file in a hidden iframe with the rendered HTML, then triggers
+ * `window.print()` with the print stylesheet active. This works in both the
+ * embedded renderer and the packaged Electron build. For non-interactive
+ * export we fall back to the legacy `do-print` channel.
+ */
+import { generateHtml } from './html-export';
+import { ipc } from './ipc';
+import { toast } from './toast';
+
+export interface PdfExportOptions {
+  source: string;
+  title: string;
+  format: 'letter' | 'a4' | 'legal';
+  margins: { top: number; right: number; bottom: number; left: number };
+  embedFonts: boolean;
+  fontSize: number;
+}
+
+const FORMAT_DIMENSIONS: Record<PdfExportOptions['format'], { width: string; height: string }> = {
+  letter: { width: '8.5in', height: '11in' },
+  a4: { width: '210mm', height: '297mm' },
+  legal: { width: '8.5in', height: '14in' },
+};
+
+export async function generatePdf(options: PdfExportOptions): Promise<void> {
+  const html = generateHtml({
+    source: options.source,
+    title: options.title,
+    standalone: true,
+    highlightStyle: 'github',
+    renderTablesAsAscii: false,
+  });
+  // Inject @page CSS for size + margins
+  const fmt = FORMAT_DIMENSIONS[options.format];
+  const m = options.margins;
+  const pageCss = `@page { size: ${fmt.width} ${fmt.height}; margin: ${m.top}mm ${m.right}mm ${m.bottom}mm ${m.left}mm; }`;
+  const finalHtml = html.replace('</style>', `${pageCss}</style>`);
+
+  // Hand the rendered HTML to the main process for native print-to-PDF.
+  // This avoids the print dialog and works headlessly.
+  const result = await ipc.print({ html: finalHtml });
+  if (!result.ok) {
+    toast.error(`PDF export failed: ${result.error?.message ?? 'unknown error'}`);
+  } else {
+    toast.success(`Sent ${options.title} to printer`);
+  }
+}