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>

frontend/styles.css

@@ -1,9 +1,120 @@
+/* Base Styles - Always Loaded */
 * {
     margin: 0;
     padding: 0;
     box-sizing: border-box;
 }
 
+:root {
+    /* CSS Variables for Style System */
+    --border-radius-small: 4px;
+    --border-radius-medium: 8px;
+    --border-radius-large: 12px;
+    --spacing-xs: 4px;
+    --spacing-sm: 8px;
+    --spacing-md: 16px;
+    --spacing-lg: 24px;
+    --spacing-xl: 32px;
+    --shadow-sm: 0 1px 3px rgba(0,0,0,0.1);
+    --shadow-md: 0 4px 6px rgba(0,0,0,0.1);
+    --shadow-lg: 0 8px 25px rgba(0,0,0,0.15);
+    --border-light: 1px solid #e9ecef;
+    --border-medium: 1px solid #dee2e6;
+    --transition-fast: 0.15s ease;
+    --transition-normal: 0.2s ease;
+    --transition-slow: 0.3s ease;
+}
+
+body {
+    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
+    background-color: #f8f9fa;
+    color: #333;
+    line-height: 1.6;
+}
+
+.app {
+    min-height: 100vh;
+    display: flex;
+    flex-direction: row;
+}
+
+.login-layout {
+    min-height: 100vh;
+    display: flex;
+    flex-direction: column;
+    width: 100%;
+}
+
+/* Base Layout */
+.main-content {
+    flex: 1;
+    margin-left: 280px;
+    overflow-x: hidden;
+}
+
+/* Basic Form Elements */
+input, select, textarea, button {
+    font-family: inherit;
+}
+
+/* Utility Classes */
+.loading {
+    opacity: 0.7;
+}
+
+.error {
+    color: #dc3545;
+}
+
+.success {
+    color: #28a745;
+}
+
+/* Theme Data Attributes for Color Schemes */
+[data-theme="default"] {
+    --primary-color: #667eea;
+    --primary-gradient: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
+}
+
+[data-theme="ocean"] {
+    --primary-color: #006994;
+    --primary-gradient: linear-gradient(135deg, #006994 0%, #0891b2 100%);
+}
+
+[data-theme="forest"] {
+    --primary-color: #065f46;
+    --primary-gradient: linear-gradient(135deg, #065f46 0%, #047857 100%);
+}
+
+[data-theme="sunset"] {
+    --primary-color: #ea580c;
+    --primary-gradient: linear-gradient(135deg, #ea580c 0%, #dc2626 100%);
+}
+
+[data-theme="purple"] {
+    --primary-color: #7c3aed;
+    --primary-gradient: linear-gradient(135deg, #7c3aed 0%, #a855f7 100%);
+}
+
+[data-theme="dark"] {
+    --primary-color: #374151;
+    --primary-gradient: linear-gradient(135deg, #374151 0%, #1f2937 100%);
+}
+
+[data-theme="rose"] {
+    --primary-color: #e11d48;
+    --primary-gradient: linear-gradient(135deg, #e11d48 0%, #f43f5e 100%);
+}
+
+[data-theme="mint"] {
+    --primary-color: #10b981;
+    --primary-gradient: linear-gradient(135deg, #10b981 0%, #059669 100%);
+}* {
+    margin: 0;
+    padding: 0;
+    box-sizing: border-box;
+}
+
 body {
     font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
     background-color: #f8f9fa;
@@ -3014,6 +3125,50 @@ body {
     padding: 0.5rem;
 }
 
+/* Style Selector Styles */
+.style-selector {
+    margin-bottom: 1rem;
+}
+
+.style-selector label {
+    display: block;
+    margin-bottom: 0.5rem;
+    font-size: 0.8rem;
+    font-weight: 600;
+    text-transform: uppercase;
+    letter-spacing: 0.5px;
+    color: rgba(255, 255, 255, 0.8);
+}
+
+.style-selector-dropdown {
+    width: 100%;
+    padding: 0.5rem;
+    background: rgba(255, 255, 255, 0.1);
+    border: 1px solid rgba(255, 255, 255, 0.2);
+    border-radius: 6px;
+    color: white;
+    font-size: 0.85rem;
+    cursor: pointer;
+    transition: all 0.2s ease;
+}
+
+.style-selector-dropdown:hover {
+    background: rgba(255, 255, 255, 0.15);
+    border-color: rgba(255, 255, 255, 0.3);
+}
+
+.style-selector-dropdown:focus {
+    outline: none;
+    background: rgba(255, 255, 255, 0.2);
+    border-color: rgba(255, 255, 255, 0.4);
+}
+
+.style-selector-dropdown option {
+    background: #333;
+    color: white;
+    padding: 0.5rem;
+}
+
 /* Theme Definitions */
 :root {
     /* Default Theme */