feat(grid): split-grid v2 — 1×3/2×3, drag-to-quadrant, Ctrl+` cycle, maximize

Builds on the v1 watch board:
- Two more layouts: row-3 (1×3) and grid-6 (2×3). A single `grid` marker class on
  #term now carries the shared cell chrome so it no longer enumerates each lay-*.
- Ctrl+` cycles the focused quadrant (Ctrl+Shift+` reverses); matchFocusCycleKey
  is an exported pure helper so it's unit-tested. No-op / passthrough in single mode.
- Per-quadrant ⛶ maximize: the focused cell expands to fill the grid as an absolute
  overlay while siblings stay live behind it; follows focus, resets on layout
  change / tab close.
- Drag a tab from the tab bar onto a quadrant to assign it there (reuses the
  existing tab-drag dragIndex); grid-only.

Adversarial review (3 lenses → per-finding verify, incl. a headless-Chrome repro)
caught a HIGH: maximizing via `grid-column/row: 1/-1` shoved siblings into implicit
rows — a strip instead of fullscreen AND a spurious resize to backgrounded live
PTYs. Fixed with an absolute-overlay (`position:absolute; inset:0`), then verified
in real Chromium (Playwright): maximized cell fills #term, siblings 0px size delta.
Also: silence a covered pane's pending pulse under maximize; coarse-pointer target
for .cell-max. Verified: typecheck + build:web clean, 1579 tests pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

public/main.ts

@@ -20,7 +20,7 @@ import { mountDashboard } from './dashboard.js'
 import { mountHistory } from './history.js'
 import { mountShortcuts } from './shortcuts.js'
 import { mountShareSession } from './share.js'
-import { mountGridToggle } from './grid-layout.js'
+import { mountGridToggle, matchFocusCycleKey } from './grid-layout.js'
 
 const paneHost = document.getElementById('term')
 const tabs = document.getElementById('tabs')
@@ -59,6 +59,21 @@ document.addEventListener('visibilitychange', () => {
   if (!document.hidden) app.refitVisible()
 })
 
+// v2: Ctrl+` cycles the focused quadrant in a split grid (Ctrl+Shift+` reverses).
+// Capture phase so xterm doesn't swallow it; left untouched (no preventDefault)
+// in single mode so the terminal keeps the keystroke.
+document.addEventListener(
+  'keydown',
+  (e) => {
+    const dir = matchFocusCycleKey(e)
+    if (dir === null) return
+    if (app.getGridLayout() === 'single') return // let the terminal keep the key
+    e.preventDefault()
+    app.cycleFocus(dir)
+  },
+  { capture: true },
+)
+
 // Toolbar utilities.
 mountSearch(toolbar, {
   find: (query, dir) => app.findInActive(query, dir),