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/chat/DiffPreview.tsx

@@ -36,7 +36,7 @@ export const DiffPreview: React.FC<DiffPreviewProps> = ({ diff, syntaxTheme, fil
                                         : {}
                             }
                         >
-                            <span className="text-muted-foreground/60 w-8 flex-shrink-0 text-right pr-2 self-start select-none">
+                            <span className="w-10 flex-shrink-0 text-right pr-3 select-none border-r mr-3 -my-0.5 py-0.5" style={{ color: 'var(--tools-edit-line-number)', borderColor: 'var(--tools-border)' }}>
                                 {line.lineNumber || ''}
                             </span>
                             <div className="flex-1 min-w-0">
@@ -94,7 +94,7 @@ export const WritePreview: React.FC<WritePreviewProps> = ({ content, syntaxTheme
             <div className="space-y-0">
                 {lines.map((line, lineIdx) => (
                     <div key={lineIdx} className="typography-code font-mono px-2 py-0.5 flex -mx-1">
-                        <span className="text-muted-foreground/60 w-8 flex-shrink-0 text-right pr-2 self-start select-none">
+                        <span className="w-10 flex-shrink-0 text-right pr-3 select-none border-r mr-3 -my-0.5 py-0.5" style={{ color: 'var(--tools-edit-line-number)', borderColor: 'var(--tools-border)' }}>
                             {lineIdx + 1}
                         </span>
                         <div className="flex-1 min-w-0">