feat(grid): v3b/c — resizable splitters + saved layout presets

Resizable splitters: draggable gutters between grid tracks adjust per-layout
column/row fractions (fr units), applied as an inline grid-template and persisted
(web-terminal:grid-splits). The fraction math (adjustSplit — trades delta between
adjacent tracks, clamps to a minimum, conserves the total) is a pure, unit-tested
helper in grid-layout.ts; the drag translates pixel motion into an fr delta.

Saved presets: public/grid-presets.ts — a toolbar dropdown to save the current
layout + its split under a name, re-apply it in one click, or delete it
(web-terminal:grid-presets). Desktop-only, like the layout toggle.

- grid-layout.ts: layoutTracks, defaultSplit, adjustSplit, tracksToTemplate,
  load/saveGridSplits, splitForLayout (validates stored shape).
- tabs.ts: renderGutters/beginGutterDrag/setSplit; applyLayout sets the inline
  template; gridArrangement()/applyGridPreset() hooks.
- main.ts: mount the presets dropdown. style.css: gutters + presets menu.
- tests: splitter math + persistence + a stubbed drag repro (1.2fr/0.8fr); presets
  persistence + dropdown (save/apply/delete/close). typecheck+build clean, 1612 pass.

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

test/tabs.test.ts

@@ -1460,4 +1460,70 @@ describe('TabApp — split-grid watch board (v1)', () => {
     app.setGridLayout('single')
     expect(monitorHandles[0]!.dispose).toHaveBeenCalled()
   })
+
+  // ── v3: resizable splitters ──────────────────────────────────────────────────
+
+  it('grid-4 renders one column + one row splitter and sets an fr template', () => {
+    const { app, paneHost } = withTabs(4)
+    app.focusTab(0)
+    app.setGridLayout('grid-4')
+    expect(paneHost.querySelectorAll('.grid-gutter-col')).toHaveLength(1)
+    expect(paneHost.querySelectorAll('.grid-gutter-row')).toHaveLength(1)
+    expect(paneHost.style.gridTemplateColumns).toBe('1fr 1fr')
+    expect(paneHost.style.gridTemplateRows).toBe('1fr 1fr')
+  })
+
+  it('grid-6 renders two column + one row splitters', () => {
+    const { app, paneHost } = withTabs(6)
+    app.focusTab(0)
+    app.setGridLayout('grid-6')
+    expect(paneHost.querySelectorAll('.grid-gutter-col')).toHaveLength(2)
+    expect(paneHost.querySelectorAll('.grid-gutter-row')).toHaveLength(1)
+  })
+
+  it('single mode has no splitters and clears the inline grid template', () => {
+    const { app, paneHost } = withTabs(3)
+    app.setGridLayout('grid-4')
+    app.setGridLayout('single')
+    expect(paneHost.querySelectorAll('.grid-gutter')).toHaveLength(0)
+    expect(paneHost.style.gridTemplateColumns).toBe('')
+  })
+
+  it('a persisted custom split is applied to the grid template on load', () => {
+    localStorage.setItem(
+      'web-terminal:grid-splits',
+      JSON.stringify({ 'grid-4': { cols: [1.5, 0.5], rows: [1, 1] } }),
+    )
+    const { paneHost, tabBar } = makeHosts()
+    const app = new TabApp(paneHost, tabBar)
+    app.newTab()
+    app.newTab()
+    app.setGridLayout('grid-4')
+    expect(paneHost.style.gridTemplateColumns).toBe('1.5fr 0.5fr')
+  })
+
+  it('dragging a column splitter re-templates the grid and persists on release', () => {
+    const { app, paneHost } = withTabs(4)
+    app.focusTab(0)
+    app.setGridLayout('grid-4')
+    // jsdom has no layout — give #term a measurable width for the drag math.
+    vi.spyOn(paneHost, 'getBoundingClientRect').mockReturnValue({
+      width: 800,
+      height: 600,
+      left: 0,
+      top: 0,
+      right: 800,
+      bottom: 600,
+      x: 0,
+      y: 0,
+      toJSON: () => ({}),
+    } as DOMRect)
+    const gutter = paneHost.querySelector<HTMLElement>('.grid-gutter-col')!
+    gutter.dispatchEvent(new MouseEvent('pointerdown', { clientX: 400, bubbles: true }))
+    gutter.dispatchEvent(new MouseEvent('pointermove', { clientX: 480, bubbles: true })) // +0.2fr
+    gutter.dispatchEvent(new MouseEvent('pointerup', { bubbles: true }))
+    expect(paneHost.style.gridTemplateColumns).toBe('1.2fr 0.8fr')
+    const saved = JSON.parse(localStorage.getItem('web-terminal:grid-splits')!)
+    expect(saved['grid-4'].cols[0]).toBeCloseTo(1.2)
+  })
 })