Add 3x3 grid layout (9-pane)

[AThraen]

This was generated by AI during triage.

Summary

Add a 3×3 grid layout option (9 terminal panes) alongside the existing 1×1, 1×2, 2×1, 2×2, and other layouts.

Motivation

Power users running many concurrent sessions — e.g. monitoring multiple Claude Code agents, tailing several log streams, or managing a fleet of services — currently max out at 4 panes (2×2). A 3×3 layout makes the tool viable for larger multi-session workflows without needing to switch groups as frequently.

Proposed behaviour

• Add ThreeByThree (or similar) to the Layout enum and the layout switcher toolbar/menu.
• RefreshTerminalLayout distributes up to 9 active sessions into a 3-column × 3-row Grid.
• Sessions beyond 9 are dormanted (same overflow behaviour as today).
• Layout persists to AppState.LastLayout as "ThreeByThree".
• The active-ring highlight and sidebar sync work unchanged — they are slot-agnostic.

Implementation hints

• The layout switcher in MainWindow toolbar already iterates layout values; adding the enum entry and a ColumnDefinitions/RowDefinitions branch in RefreshTerminalLayout should be the bulk of the work.
• Minimum window size may need revisiting — 9 panes at the current minimum will be very cramped; consider enforcing a larger MinWidth/MinHeight when this layout is active, or letting xterm.js reflow naturally via the fit-addon.
• The layout label/icon in the toolbar needs a 3×3 grid glyph or label.