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>

README.md

@@ -47,9 +47,11 @@ The core workflow:
 
 - **Guide library** with folders, favorites, title search, full-text search,
   duplicate/move/delete, and a quick-actions palette (`Ctrl+/`).
-- **Capture engine** — full screen, active window, and region capture with
-  delay, pause/resume, hotkeys, click markers, clipboard paste, and PNG/JPEG/
-  GIF import.
+- **Capture engine** — the editor's **Capture ▾** button offers full screen,
+  active window, and region capture (the app hides itself during the shot)
+  plus delay, pause/resume sessions with global hotkeys, click markers,
+  clipboard paste, and PNG/JPEG/GIF import. The full keyboard shortcut list
+  lives under **More ▾ → Keyboard shortcuts** in the editor.
 - **Three-pane editor** — step tree with substeps, statuses
   (todo/in-progress/done), hidden/skipped steps, focused view (zoom/pan that
   never mutates the original image), autosave, and command-stack undo/redo.