refactor(ui): unify context menus into a shared menu system

Five hand-rolled context menu implementations (image tiles, sidebar
folders, sidebar albums, title bar theme switcher, plus duplicated
close-listener effects in Gallery/Timeline) are replaced by shared
primitives in src/components/menu/:

- useDismissable: one outside-pointerdown + Escape dismissal hook
- Menu.tsx: MenuPanel chrome, MenuItem (danger/disabled/checked/hint),
  MenuSeparator, MenuLabel, and SubMenu with viewport edge-flipping
- ContextMenu: portal-rendered wrapper that measures and clamps to the
  viewport, fixing menus rendering off-screen and the latent
  fixed-inside-transform bug under framer-motion Reorder items

The image right-click menu moves to ImageContextMenu.tsx, shared by
Gallery and Timeline.

CHANGELOG.md

@@ -59,6 +59,9 @@ aims to follow [Semantic Versioning](https://semver.org/spec/v2.0.0.html)
 
 ### Changed
 
+- **Consistent right-click menus** — the folder, album, image, and theme menus
+  now share one look and behavior: they stay inside the window instead of
+  spilling off the edge, close on Escape, and support proper submenus.
 - **Neater lightbox details** — image and video metadata now sits in two columns,
   so the info panel shows more at a glance with less scrolling.
 - **Faster Explore revisits** — returning to a folder's visual clusters should