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/renderer/app.js

@@ -265,12 +265,30 @@ class StepForgeApp {
     const guide = this.editorMeta?.guide;
     this.topbarContext.append(
       el('button', { type: 'button', onClick: () => this.showLibrary() }, 'Back'),
-      el('button', { type: 'button', onClick: () => this.renameGuide() }, 'Rename'),
+      el('button.primary', {
+        type: 'button',
+        title: 'Capture a screenshot step',
+        onClick: (e) => this.editor.openCaptureMenu(e),
+      }, 'Capture ▾'),
       el('button', { type: 'button', onClick: () => this.editor.saveAll() }, 'Save'),
       el('button', { type: 'button', onClick: () => this.editor.openExportDialog() }, 'Export'),
-      el('button', { type: 'button', onClick: () => this.editor.openLinkedGuide() }, guide && guide.linkedSource ? 'Linked' : 'Local'),
-      el('button', { type: 'button', onClick: () => this.editor.openQuickActions() }, 'Quick'),
-      el('button', { type: 'button', onClick: () => this.openSettings() }, 'Settings'),
+      el('button', { type: 'button', title: 'Share this guide as a .sfgz file', onClick: () => this.editor.shareAsFile() }, 'Share'),
+      el('button', {
+        type: 'button',
+        onClick: (e) => {
+          const rect = e.target.getBoundingClientRect();
+          contextMenu(rect.left, rect.bottom + 4, [
+            { label: 'Rename guide…', action: () => this.renameGuide() },
+            { label: 'Guide placeholders…', action: () => this.editor.openGuidePlaceholders() },
+            { label: 'Backups & snapshots…', action: () => this.editor.openBackupsDialog() },
+            { label: guide && guide.linkedSource ? 'Linked guide…' : 'Linked guide (not linked)', action: () => this.editor.openLinkedGuide() },
+            'sep',
+            { label: 'Keyboard shortcuts…', action: () => this.editor.openShortcutsHelp() },
+            { label: 'Quick actions  (Ctrl+/)', action: () => this.editor.openQuickActions() },
+            { label: 'Settings…', action: () => this.openSettings() },
+          ]);
+        },
+      }, 'More ▾'),
       el('span.muted', { style: { marginLeft: '8px' } }, guide ? `${guide.title} · ${this.editorMeta?.stepCount || 0} steps` : ''),
     );
   }