Complete the app: capture UI, dialogs, template manager, shortcuts help

- Editor topbar reworked: Back | Capture ▾ (full screen/window/region/
  delay/paste/import/session) | Save | Export | Share (.sfgz) | More ▾
  (rename, guide placeholders, backups, linked guide, shortcuts,
  settings)
- New dialogs: backups & snapshots (undoable restore), guide/global
  placeholder editor, keyboard-shortcuts reference, template manager
  (rename/duplicate/delete/share/import .sfglt)
- Export dialog: editable per-format options generated from exporter
  defaults, save-as-template, preview opens the file in the default
  viewer and keeps the dialog open for tweaking
- export:defaults IPC + preload entry
- CSS for blocks panel, focused-view sliders, export options, rows
- ipc-surface test: every preload channel has a main handler; renderer
  api.*/dialogs.* usage stays within the exposed surface (60 tests)
- CHANGELOG/README updated; prompt2.md checklist fully ticked

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/main.js

@@ -348,6 +348,23 @@ function setupIpc() {
 
   // export + preview
   h('export:formats', () => FORMATS.filter((f) => EXPORTERS[f]));
+  h('export:defaults', ({ format }) => {
+    // Exporter modules expose DEFAULT_TEMPLATE; the dialog renders editable
+    // options from it (booleans -> checkbox, numbers -> number, strings -> text).
+    const mod = {
+      json: '../exporters/json',
+      markdown: '../exporters/markdown',
+      'html-simple': '../exporters/html',
+      'html-rich': '../exporters/html',
+      pdf: '../exporters/pdf',
+      gif: '../exporters/gif',
+      'image-bundle': '../exporters/image-bundle',
+      docx: '../exporters/docx',
+      pptx: '../exporters/pptx',
+    }[format];
+    if (!mod) return {};
+    return { ...require(mod).DEFAULT_TEMPLATE };
+  });
   h('export:run', async ({ guideId, format, options, outDir }) => {
     let dir = outDir || settings.get(`exports.lastOutputDirs.${format}`);
     if (!dir) {