style(header): hover moves the icon, never the button

A global `.btn-icon-header:hover { transform: rotate(45deg) }`, meant for
the settings gear, turned every header icon button on hover, so the folder,
Tiles, Split and the rest swung their rounded hover background into a
diamond. Three buttons had already cancelled it one by one (the font-size
buttons, notifications, the sidebar toggle).

The rule is gone, and with it those three overrides. Hover motion now moves
the icon only:
- the settings gear's icon turns 45 degrees (one tooth, so it lands on the
  same shape);
- the Tiles button's four squares spread apart, each toward its corner;
- the folder cross-fades to an open folder (a second drawing in its SVG,
  `.icon-folder-closed` / `.icon-folder-open`);
- every other icon just takes the hover colour.

Pointer devices only (`@media (hover: hover)`, so a tap cannot leave an icon
stuck mid-motion), and the transitions are off under reduced motion.

Owner request: the Tiles and folder buttons "weirdly turn" on hover.
Checked live on a dark and a light skin (rest, mid, end frames). Pinned by
test/header-icon-hover.test.ts, mutation-checked five ways (the button
rotation back, the open drawing missing, the motion not hover-gated, a
square spreading toward the wrong corner, reduced motion keeping its
transition). Gate: 507 files, 9798 tests passed.

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

docs/tile-grid-plan.md

@@ -68,6 +68,15 @@ or settled a question the spec left open. The invariants as built are in
   every joining tile mounted and laid out before any connects (one fit, one PTY resize
   each), and a zoom the user chose ends. The other ways in (Ctrl/Cmd+click, a dragged tab,
   "Open group as tiles", Run) still add up to the cap of 6.
+- **Header icons move their icon on hover, never the button** (owner request: the Tiles
+  and folder buttons "weirdly turn" on hover; make a nicer hover). A global
+  `.btn-icon-header:hover { transform: rotate(45deg) }` (meant for the settings gear)
+  turned every header icon button, swinging its hover background into a diamond. Now
+  only the gear's ICON turns (45 degrees, one tooth), the Tiles button's four squares
+  spread apart, and the folder cross-fades to an open folder (`.icon-folder-closed` /
+  `.icon-folder-open` in its SVG); every other icon just takes the hover colour. Pointer
+  devices only (`@media (hover: hover)`), transitions off under reduced motion. Pinned by
+  `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, not an `entrance-animations.js` theme (those are off by default). Opening, each tile