Implement comprehensive style system with Google Calendar theme

This commit adds a complete style system alongside the existing theme system, allowing users to switch between different UI styles while maintaining theme color variations.

**Core Features:**
- Style enum (Default, Google Calendar) separate from Theme enum
- Hot-swappable stylesheets with dynamic loading
- Style preference persistence (localStorage + database)
- Style picker UI in sidebar below theme picker

**Frontend Implementation:**
- Add Style enum to sidebar.rs with value/display methods
- Implement dynamic stylesheet loading in app.rs
- Add style picker dropdown with proper styling
- Handle style state management and persistence
- Add web-sys features for HtmlLinkElement support

**Backend Integration:**
- Add calendar_style column to user_preferences table
- Update all database operations (insert/update/select)
- Extend API models for style preference
- Add migration for existing users

**Google Calendar Style:**
- Clean Material Design-inspired interface
- White sidebar with proper contrast
- Enhanced calendar grid with subtle shadows
- Improved event styling with hover effects
- Google Sans typography throughout
- Professional color scheme and spacing

**Technical Details:**
- Trunk asset management for stylesheet copying
- High CSS specificity to override theme styles
- Modular CSS architecture for easy extensibility
- Comprehensive text contrast fixes
- Enhanced calendar cells and navigation

Users can now choose between the original gradient design (Default) and a clean Google Calendar-inspired interface (Google Calendar), with full preference persistence across sessions.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/migrations/004_add_style_preference.sql

@@ -0,0 +1,2 @@
+-- Add calendar style preference to user preferences
+ALTER TABLE user_preferences ADD COLUMN calendar_style TEXT DEFAULT 'default';