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>

frontend/src/locales/en.json

@@ -271,6 +271,15 @@
       "roboto": "Roboto",
       "robotoDesc": "Google's classic"
     },
+    "tabBarPosition": "Tab Position",
+    "tabBarPositions": {
+      "top": "Top",
+      "topDesc": "Tab bar always on top",
+      "bottom": "Bottom",
+      "bottomDesc": "Tab bar always on bottom",
+      "responsive": "Responsive",
+      "responsiveDesc": "Top on desktop, bottom on mobile"
+    },
     "palettes": {
       "default": "Default",
       "defaultDesc": "Standard modern palette",