Add theme customization system and theme manager

Introduces a full browser customization system with support for theme presets, live editing, import/export, and non-destructive design. Adds new documentation for customization, updates the features list, and implements a ThemeManager for loading and managing themes at the application level. Updates home and settings pages to support live theming and preview, and adds four built-in themes (default, forest, ocean, sunset).

themes/default.json

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+{
+  "name": "Default Nebula",
+  "colors": {
+    "bg": "#121418",
+    "darkBlue": "#0B1C2B",
+    "darkPurple": "#1B1035",
+    "primary": "#7B2EFF",
+    "accent": "#00C6FF",
+    "text": "#E0E0E0"
+  },
+  "layout": "centered",
+  "showLogo": true,
+  "customTitle": "Nebula Browser",
+  "gradient": "linear-gradient(145deg, #121418 0%, #1B1035 100%)",
+  "version": "1.0",
+  "description": "The original Nebula Browser theme with purple and blue gradients"
+}

themes/forest.json

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+{
+  "name": "Forest Night",
+  "colors": {
+    "bg": "#1a202c",
+    "darkBlue": "#2d3748",
+    "darkPurple": "#4a5568",
+    "primary": "#68d391",
+    "accent": "#9ae6b4",
+    "text": "#f7fafc"
+  },
+  "layout": "centered",
+  "showLogo": true,
+  "customTitle": "Nebula Browser",
+  "gradient": "linear-gradient(145deg, #1a202c 0%, #2d3748 100%)",
+  "version": "1.0",
+  "description": "A nature-inspired theme with forest greens and earth tones"
+}

themes/ocean.json

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+{
+  "name": "Ocean Depths",
+  "colors": {
+    "bg": "#1a365d",
+    "darkBlue": "#2a4365",
+    "darkPurple": "#2c5282",
+    "primary": "#3182ce",
+    "accent": "#00d9ff",
+    "text": "#e2e8f0"
+  },
+  "layout": "centered",
+  "showLogo": true,
+  "customTitle": "Nebula Browser",
+  "gradient": "linear-gradient(145deg, #1a365d 0%, #2c5282 100%)",
+  "version": "1.0",
+  "description": "A calming ocean-inspired theme with blues and teals"
+}

themes/sunset.json

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+{
+  "name": "Sunset Glow",
+  "colors": {
+    "bg": "#744210",
+    "darkBlue": "#975a16",
+    "darkPurple": "#c05621",
+    "primary": "#ed8936",
+    "accent": "#fbb040",
+    "text": "#fffaf0"
+  },
+  "layout": "centered",
+  "showLogo": true,
+  "customTitle": "Nebula Browser",
+  "gradient": "linear-gradient(145deg, #744210 0%, #c05621 100%)",
+  "version": "1.0",
+  "description": "A warm sunset theme with oranges and golden hues"
+}