feat: introduced a token-based theming system across the UI

* feat: added themes system

* feat: smart sidebar auto-hide for files/diff tabs + lower files sidebar threshold

* feat: added Checkbox component and update theming

- Add reusable Checkbox component for toggles across UI
- Replace several inputs with Checkbox in settings and commands panels
- Add DiffIcon and apply surface/border theming to key UI areas

* feat: Add convert-vscode-theme.cjs to convert VS Code themes to OpenChamber format

* refactor: remove unused permission logic from ChatInput

- Remove unused permission rules parsing logic from ChatInput
- Memoize renderTheme in DiffWorkerProvider to avoid unnecessary recalculations
- Remove forceOpaque helper in vscode theme adapter

* fix: guard VSCode theme loading in MarkdownRenderer

* feat: add custom user themes loading and reload

- Load user themes from ~/.config/openchamber/themes at runtime
- Expose /api/config/themes to fetch custom themes
- Allow theme reloading from Settings → Theme → Reload themes in the UI

packages/ui/src/components/sections/shared/SettingsSidebarItem.tsx

@@ -70,8 +70,8 @@ export const SettingsSidebarItem: React.FC<SettingsSidebarItemProps> = ({
       className={cn(
         'group relative flex items-center rounded-md px-1.5 py-1 transition-all duration-200',
         selected
-          ? 'dark:bg-accent/80 bg-primary/12'
-          : 'hover:dark:bg-accent/40 hover:bg-primary/6',
+          ? 'bg-interactive-selection'
+          : 'hover:bg-interactive-hover',
         className
       )}
     >