Add workspace presets for different usage modes. Each workspace stores
layout mode + column configuration. Switching destroys current layout
and rebuilds from saved config (no background state).
- Add Workspace data model with WorkspaceColumn
- Add WorkspaceManager with save/load/switch/add/delete
- Add WorkspaceIcons registry for Material icon resolution
- Add WorkspaceBar to AppDrawer footer with workspace chips
- Add DeckState.loadFromWorkspace() for column rebuilding
- Add workspaces persistence to DesktopPreferences
- Default workspace: "Social" (Home + Notifications + DMs)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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