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/icons/DiffIcon.tsx

@@ -0,0 +1,27 @@
+import type { SVGProps } from 'react';
+
+interface DiffIconProps extends Omit<SVGProps<SVGSVGElement>, 'children'> {
+  size?: number | string;
+}
+
+/**
+ * Git merge/branch icon for the Diff tab.
+ */
+export function DiffIcon({ size, className, style, ...props }: DiffIconProps) {
+  return (
+    <svg
+      xmlns="http://www.w3.org/2000/svg"
+      viewBox="0 0 256 256"
+      fill="currentColor"
+      className={className}
+      style={{
+        width: typeof size === 'number' ? `${size}px` : size,
+        height: typeof size === 'number' ? `${size}px` : size,
+        ...style,
+      }}
+      {...props}
+    >
+      <path d="M112,148a12,12,0,0,0-12,12v19L69.17,148.2A4,4,0,0,1,68,145.37V97.94a36,36,0,1,0-24,0v47.43a27.81,27.81,0,0,0,8.2,19.8L83,196H64a12,12,0,0,0,0,24h48a12,12,0,0,0,12-12V160A12,12,0,0,0,112,148ZM56,52A12,12,0,1,1,44,64,12,12,0,0,1,56,52ZM212,158.06V110.63a27.81,27.81,0,0,0-8.2-19.8L173,60h19a12,12,0,0,0,0-24H144a12,12,0,0,0-12,12V96a12,12,0,0,0,24,0V77l30.83,30.83a4,4,0,0,1,1.17,2.83v47.43a36,36,0,1,0,24,0ZM200,204a12,12,0,1,1,12-12A12,12,0,0,1,200,204Z" />
+    </svg>
+  );
+}