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/views/agent-manager/AgentManagerSidebar.tsx

@@ -74,7 +74,7 @@ const AgentGroupItem: React.FC<AgentGroupItemProps> = ({ group, isSelected, onSe
       <div
         className={cn(
           'group relative flex items-center rounded-md px-1.5 py-1.5 cursor-pointer',
-          isSelected ? 'dark:bg-accent/80 bg-primary/12' : 'hover:dark:bg-accent/40 hover:bg-primary/6',
+          isSelected ? 'bg-interactive-selection' : 'hover:bg-interactive-hover',
         )}
         onClick={onSelect}
       >
@@ -192,7 +192,7 @@ export const AgentManagerSidebar: React.FC<AgentManagerSidebarProps> = ({
   const remainingCount = filteredGroups.length - MAX_VISIBLE;
   
   return (
-    <div className={cn('flex h-full flex-col bg-background/50 dark:bg-neutral-900/80 text-foreground border-r border-border/30', className)}>
+    <div className={cn('flex h-full flex-col text-foreground border-r border-border/30', className)}>
       {/* Search Input */}
       <div className="px-2.5 pt-3 pb-2">
         <div className="relative">