refactor(layouts): simplify to global config + preserve windows + add UI dropdown

Simplified layout system based on user feedback:

**1. Global Layout Config (Simpler)**
- Moved layoutConfig from per-workspace to global state
- One configuration applies to all workspaces (easier to understand)
- Updated state migration v8→v9 to move config to global level
- Updated WorkspaceSettings UI to edit global config
- Renamed updateWorkspaceLayoutConfig → updateLayoutConfig

**2. Preserve Extra Windows (Fixed Bug)**
- Fixed applyPresetToLayout to keep windows beyond preset slots
- When applying 4-window grid to 6 windows, windows 5-6 are preserved
- Extra windows stacked vertically on right side (70/30 split)
- No more window loss when applying presets

**3. Layout Dropdown in TabBar (Better UX)**
- Added dropdown menu next to workspace tabs
- Shows all available presets with icons (Grid2X2, Columns2, Split)
- Displays window requirements and availability
- Disables presets that need more windows than available
- One-click preset application with toast feedback
- More accessible than /layout command

All tests passing (457 passed). State migration handles v6→v7→v8→v9 correctly.

Generated with [Claude Code](https://claude.com/claude-code)

src/types/app.ts

@@ -65,7 +65,6 @@ export interface Workspace {
   label?: string; // Optional user-editable label
   layout: MosaicNode<string> | null;
   windowIds: string[];
-  layoutConfig: LayoutConfig; // How new windows are inserted into layout
 }
 
 export interface RelayInfo {
@@ -85,6 +84,7 @@ export interface GrimoireState {
   windows: Record<string, WindowInstance>;
   workspaces: Record<string, Workspace>;
   activeWorkspaceId: string;
+  layoutConfig: LayoutConfig; // Global configuration for window insertion (applies to all workspaces)
   activeAccount?: {
     pubkey: string;
     relays?: UserRelays;