feat(settings): an Animations section in App Settings

Every animation setting now has its own App Settings section, right after
Appearance (owner: easier to find). It holds the Entrance Theme (the former
Entrance Animations row), Tile Animations, and an Open lab button that closes
settings and opens the per-surface lab (?animlab=1). Appearance keeps the skin,
identity and tab settings. New animation settings go in this section;
test/app-settings-structure.test.ts pins it.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/tile-grid-plan.md

@@ -97,7 +97,7 @@ or settled a question the spec left open. The invariants as built are in
   `test/header-icon-hover.test.ts`.
 - **The grid opens and closes with a short animation, on by default** (owner request:
   "when clicking on the tile button first make this animation nicer"). It is the grid's
-  own `settle` style, the default of App Settings → Appearance → Tile Animations, which
+  own `settle` style, the default of App Settings → Animations → Tile Animations, which
   switches on other styles (`fly` out of the tabs, `deal` from the Tiles button, `crt`,
   `beam`, ...; docs/architecture-invariants.md#entrance-animations).
   Opening, each tile