feat: add UI customization and model management features (#60)

* feat: add UI customization and model management features

Add comprehensive UI customization options and enhanced model selection:

- **Favorite & Recent Models**: Star models for quick access, track 5 most recent
  - Add favorite/recent sections to model dropdowns
  - Persist preferences in local storage
  - Works in ModelControls and ModelSelector components

- **Tool Call Expansion Settings**: Control default expansion state for tool outputs
  - Three modes: collapsed, activity (summary), detailed
  - Applies to activity groups and individual tool calls
  - Configurable in Appearance Settings

- **Font Size & Spacing Controls**: Adjustable typography and layout density
  - Font size: 50-200% scaling of all semantic typography
  - Spacing/padding: 50-200% scaling of margins, gaps, line heights
  - Real-time preview with reset buttons
  - Settings in Appearance Settings

- **VSCode Extension Settings View**: Full settings access in VSCode extension
  - Add settings navigation and view type
  - Settings button in header
  - Navigate between sessions, chat, and settings

Technical changes:
- Enhanced useUIStore with new state management
- Dynamic CSS variable scaling for typography and spacing
- Typography helper functions for variable access
- ThemeProvider integration for auto-applying scales

* fix: hide theme mode in VSCode and fix detailed tool expansion

- Hide "Theme Mode" setting in VSCode extension settings as VSCode
  dynamically applies its own theme to the extension
- Fix bug where tools from subsequent messages in a turn weren't
  expanded when "Detailed" mode was selected
- Now correctly aggregates all tool IDs from turnGroupingContext when
  calculating effective expanded tools for progressive groups

packages/ui/src/lib/typography.ts

@@ -7,6 +7,28 @@ export const SEMANTIC_TYPOGRAPHY = {
   micro: '0.875rem',
 } as const;
 
+export const FONT_SIZE_SCALES = {
+  small: {
+    markdown: '0.875rem',
+    code: '0.8125rem',
+    uiHeader: '0.875rem',
+    uiLabel: '0.8125rem',
+    meta: '0.8125rem',
+    micro: '0.75rem',
+  },
+  medium: SEMANTIC_TYPOGRAPHY,
+  large: {
+    markdown: '1rem',
+    code: '0.9375rem',
+    uiHeader: '1rem',
+    uiLabel: '0.9375rem',
+    meta: '0.9375rem',
+    micro: '0.9375rem',
+  },
+} as const;
+
+export type FontSizeOption = keyof typeof FONT_SIZE_SCALES;
+
 export const VSCODE_TYPOGRAPHY = {
   // Keep VS Code webview typography slightly tighter; VS Code UI chrome already provides density.
   markdown: '0.9063rem',