fix: add custom accent color picker to Preferences page

The Preferences page had its own inline accent color section that wasn't
updated with the custom picker. Now shows:
- 5 preset swatches + separator + custom swatch with pipette icon
- Color picker + hex input appear when custom is selected
- Larger swatches (w-8 h-8) matching the page's spacious layout

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/pages/Preferences.tsx

@@ -1,4 +1,4 @@
-import { Sun, Monitor, Moon } from 'lucide-react'
+import { Sun, Monitor, Moon, Pipette } from 'lucide-react'
 import { clsx } from 'clsx'
 import { useThemeStore, ACCENT_PRESETS, type ThemeMode } from '../store/theme'
 
@@ -9,7 +9,7 @@ const MODES: { key: ThemeMode; icon: typeof Sun; label: string }[] = [
 ]
 
 export default function PreferencesPage() {
-  const { mode, accent, setMode, setAccent } = useThemeStore()
+  const { mode, accent, customHex, setMode, setAccent, setCustomHex } = useThemeStore()
 
   return (
     <div className="p-8 max-w-2xl">
@@ -54,7 +54,8 @@ export default function PreferencesPage() {
             <p className="text-sm font-medium text-content">Accent color</p>
             <p className="text-xs text-content-muted mt-0.5">Used for buttons, links, and highlights</p>
           </div>
-          <div className="flex gap-3 pt-1">
+          <div className="flex items-center gap-3 pt-1 flex-wrap">
+            {/* Preset swatches */}
             {ACCENT_PRESETS.map(({ key, label, hex }) => (
               <button
                 key={key}
@@ -71,6 +72,57 @@ export default function PreferencesPage() {
                 }}
               />
             ))}
+
+            {/* Separator */}
+            <div className="w-px h-6 bg-border-default" />
+
+            {/* Custom color swatch */}
+            <button
+              onClick={() => setAccent('custom')}
+              title="Custom color"
+              className={clsx(
+                'w-8 h-8 rounded-full transition-all flex items-center justify-center border',
+                accent === 'custom' ? 'scale-125' : 'hover:scale-110',
+              )}
+              style={{
+                backgroundColor: customHex,
+                borderColor: accent === 'custom' ? 'transparent' : 'var(--color-border)',
+                outline: accent === 'custom' ? `2px solid ${customHex}` : undefined,
+                outlineOffset: accent === 'custom' ? '3px' : undefined,
+              }}
+            >
+              {accent !== 'custom' && (
+                <Pipette size={14} className="text-white drop-shadow-sm" />
+              )}
+            </button>
+
+            {/* Color picker + hex input (when custom is active) */}
+            {accent === 'custom' && (
+              <div className="flex items-center gap-2">
+                <input
+                  type="color"
+                  value={customHex}
+                  onChange={(e) => setCustomHex(e.target.value)}
+                  className="w-8 h-8 rounded-lg border border-border-default cursor-pointer p-0"
+                  title="Pick a custom accent color"
+                />
+                <input
+                  type="text"
+                  value={customHex}
+                  onChange={(e) => {
+                    const v = e.target.value
+                    if (/^#[0-9a-fA-F]{6}$/.test(v)) setCustomHex(v)
+                  }}
+                  onBlur={(e) => {
+                    let v = e.target.value
+                    if (!v.startsWith('#')) v = '#' + v
+                    if (/^#[0-9a-fA-F]{6}$/.test(v)) setCustomHex(v)
+                  }}
+                  className="w-24 px-2 py-1.5 text-sm font-mono border border-border-default rounded-lg bg-surface text-content focus:outline-none focus:border-accent"
+                  placeholder="#6366f1"
+                />
+              </div>
+            )}
           </div>
         </div>
       </section>