Problem

v3 caps panes at 3 in a horizontal flex row. Power users with ultrawide monitors want 4–6 panes, ideally in a configurable grid (e.g. 2x2 or 3x2).

Proposed approach

• Replace the flex row layout with CSS Grid.
• Settings option for "Layout": 1, 2, 3, 4 (2x2), 6 (3x2).
• Each cell gets its own webview, all sharing the same session partition.

Files to touch

• `src/index.html` — grid CSS
• `src/renderer.js` — paneManager refactor

Acceptance criteria

• 2x2 grid renders four NotebookLM panes.
• Per-pane error overlays still work.
• Active-pane tracking still routes Quick-Clip correctly.