Add tab bar position setting with edge swipe sidebar

- Add theme_tab_bar_position setting (top/bottom/responsive)
- Tab bar is now fixed at top, stays visible during scroll
- Bottom position uses fixed positioning with safe-area-inset
- Add edge swipe gesture to open sidebar on mobile
- Remove backdrop-filter for better scroll performance
- Simplify TabsScroller by removing inline style manipulation

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/core/settings_registry.py

@@ -210,6 +210,17 @@ register_setting(SettingDefinition(
     category="theme"
 ))
 
+register_setting(SettingDefinition(
+    key="theme_tab_bar_position",
+    type=SettingType.STRING,
+    scope=SettingScope.GLOBAL,
+    storage=SettingStorage.DATABASE,
+    default="top",
+    description="Position of the tab bar (top, bottom, or responsive)",
+    category="theme",
+    choices=["top", "bottom", "responsive"]
+))
+
 
 # =============================================================================
 # MODULE/FEATURE SETTINGS (Global, Database)