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/src/db.rs

@@ -93,6 +93,7 @@ pub struct UserPreferences {
     pub calendar_time_increment: Option<i32>,
     pub calendar_view_mode: Option<String>,
     pub calendar_theme: Option<String>,
+    pub calendar_style: Option<String>,
     pub calendar_colors: Option<String>, // JSON string
     pub updated_at: DateTime<Utc>,
 }
@@ -106,6 +107,7 @@ impl UserPreferences {
             calendar_time_increment: Some(15),
             calendar_view_mode: Some("month".to_string()),
             calendar_theme: Some("light".to_string()),
+            calendar_style: Some("default".to_string()),
             calendar_colors: None,
             updated_at: Utc::now(),
         }
@@ -264,14 +266,15 @@ impl<'a> PreferencesRepository<'a> {
             sqlx::query(
                 "INSERT INTO user_preferences 
                  (user_id, calendar_selected_date, calendar_time_increment, 
-                  calendar_view_mode, calendar_theme, calendar_colors, updated_at) 
-                 VALUES (?, ?, ?, ?, ?, ?, ?)",
+                  calendar_view_mode, calendar_theme, calendar_style, calendar_colors, updated_at) 
+                 VALUES (?, ?, ?, ?, ?, ?, ?, ?)",
             )
             .bind(&prefs.user_id)
             .bind(&prefs.calendar_selected_date)
             .bind(&prefs.calendar_time_increment)
             .bind(&prefs.calendar_view_mode)
             .bind(&prefs.calendar_theme)
+            .bind(&prefs.calendar_style)
             .bind(&prefs.calendar_colors)
             .bind(&prefs.updated_at)
             .execute(self.db.pool())
@@ -286,7 +289,7 @@ impl<'a> PreferencesRepository<'a> {
         sqlx::query(
             "UPDATE user_preferences 
              SET calendar_selected_date = ?, calendar_time_increment = ?, 
-                 calendar_view_mode = ?, calendar_theme = ?, 
+                 calendar_view_mode = ?, calendar_theme = ?, calendar_style = ?,
                  calendar_colors = ?, updated_at = ?
              WHERE user_id = ?",
         )
@@ -294,6 +297,7 @@ impl<'a> PreferencesRepository<'a> {
         .bind(&prefs.calendar_time_increment)
         .bind(&prefs.calendar_view_mode)
         .bind(&prefs.calendar_theme)
+        .bind(&prefs.calendar_style)
         .bind(&prefs.calendar_colors)
         .bind(Utc::now())
         .bind(&prefs.user_id)