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/components/editor/CodeMirrorEditor.tsx

@@ -1,4 +1,4 @@
-import { useEffect, useRef } from 'react';
+import { useEffect, useRef, useState } from 'react';
 import { EditorState, Compartment } from '@codemirror/state';
 import { EditorView, keymap, lineNumbers, highlightActiveLine, drawSelection } from '@codemirror/view';
 import { defaultKeymap, history, historyKeymap, indentWithTab } from '@codemirror/commands';
@@ -10,6 +10,7 @@ import { useTheme } from 'next-themes';
 import { lightTheme, lightHighlight } from './themes/light';
 import { useEditorStore } from '@/stores/editor-store';
 import { useSettingsStore } from '@/stores/settings-store';
+import { setActiveView } from '@/lib/editor-commands';
 import { Minimap } from './Minimap';
 
 interface Props {
@@ -27,6 +28,11 @@ export function CodeMirrorEditor({ bufferId, initialContent, onChange, onCursorC
   const updateContent = useEditorStore((s) => s.updateContent);
   const setCursor = useEditorStore((s) => s.setCursor);
   const minimap = useSettingsStore((s) => s.minimap);
+  const editorFontSize = useSettingsStore((s) => s.editorFontSize);
+  // Live scroll metrics for the minimap. `0..1` ratios; defaults match
+  // Minimap's static defaults so it renders before any scroll event.
+  const [scrollRatio, setScrollRatio] = useState(0);
+  const [visibleRatio, setVisibleRatio] = useState(1);
 
   useEffect(() => {
     if (!ref.current) return;
@@ -58,11 +64,27 @@ export function CodeMirrorEditor({ bufferId, initialContent, onChange, onCursorC
             onCursorChange?.(lineNo, col);
           }
         }),
+        EditorView.theme({
+          '&': { fontSize: `${editorFontSize}px` },
+        }),
+        EditorView.domEventHandlers({
+          scroll(_event, view) {
+            const el = view.scrollDOM;
+            const denom = el.scrollHeight - el.clientHeight;
+            setScrollRatio(denom > 0 ? el.scrollTop / denom : 0);
+            setVisibleRatio(
+              el.clientHeight > 0 ? Math.min(1, el.clientHeight / el.scrollHeight) : 1,
+            );
+            return false;
+          },
+        }),
       ],
     });
     const view = new EditorView({ state, parent: ref.current });
     viewRef.current = view;
+    setActiveView(view);
     return () => {
+      setActiveView(null);
       view.destroy();
       viewRef.current = null;
     };
@@ -82,7 +104,7 @@ export function CodeMirrorEditor({ bufferId, initialContent, onChange, onCursorC
   return (
     <div className="relative h-full overflow-hidden">
       <div ref={ref} className="h-full overflow-hidden" />
-      {minimap && <Minimap content={initialContent} />}
+      {minimap && <Minimap content={initialContent} scrollRatio={scrollRatio} visibleRatio={visibleRatio} />}
     </div>
   );
-}
+}