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>

vitest.config.ts

@@ -25,6 +25,7 @@ export default defineConfig({
         'public/terminal-session.ts',
         'public/tabs.ts',
         'public/grid-layout.ts',
+        'public/grid-presets.ts',
         'public/cell-monitor.ts',
         'public/preview-grid.ts',
         'public/title-util.ts',