fix: Dark mode headings, settings redirect, upload nonce, mobile theme toggle

## 1. Fix Dark Mode Headings ✅

**Issue:** h1-h6 headings not changing color in dark mode

**Fix:**
```css
h1, h2, h3, h4, h5, h6 { @apply text-foreground; }
```

**Result:** All headings now use foreground color (adapts to theme)

---

## 2. Fix Settings Default Route ✅

**Issue:** Main Settings menu goes to /settings with placeholder page

**Fix:**
- Changed /settings to redirect to /settings/store
- Store Details is now the default settings page
- No more placeholder "Settings interface coming soon"

**Code:**
```tsx
useEffect(() => {
  navigate('/settings/store', { replace: true });
}, [navigate]);
```

---

## 3. Fix "Cookie check failed" Upload Error ✅

**Issue:** Image upload failing with "Cookie check failed"

**Root Cause:** WordPress REST API nonce not available

**Fix:**
- Added `wpApiSettings` to both dev and prod modes
- Provides `root` and `nonce` for WordPress REST API
- Image upload component already checks multiple nonce sources

**Backend Changes:**
```php
// Dev mode
wp_localize_script($handle, 'wpApiSettings', [
    'root'  => esc_url_raw(rest_url()),
    'nonce' => wp_create_nonce('wp_rest'),
]);

// Prod mode (same)
```

**Result:** Image upload now works with proper authentication

---

## 4. Add Theme Toggle to Mobile ✅

**Recommendation:** Yes, mobile should have theme toggle

**Implementation:** Added to More page (mobile hub)

**UI:**
- 3-column grid with theme cards
- ☀️ Light | 🌙 Dark | 🖥️ System
- Active theme highlighted with primary border
- Placed under "Appearance" section

**Location:**
```
More Page
├── Coupons
├── Settings
├── Appearance (NEW)
│   ├── ☀️ Light
│   ├── 🌙 Dark
│   └── 🖥️ System
└── Exit Fullscreen / Logout
```

**Why More page?**
- Mobile users go there for additional options
- Natural place for appearance settings
- Doesn't clutter main navigation
- Desktop has header toggle, mobile has More page

---

## Summary

✅ **Dark mode headings** - Fixed with text-foreground
✅ **Settings redirect** - /settings → /settings/store
✅ **Upload nonce** - wpApiSettings added (dev + prod)
✅ **Mobile theme toggle** - Added to More page with 3-card grid

**All issues resolved!** 🎉

**Note:** CSS lint warnings (@tailwind, @apply) are false positives - Tailwind directives are valid.

admin-spa/src/index.css

@@ -65,6 +65,7 @@
 @layer base {
   * { @apply border-border; }
   body { @apply bg-background text-foreground; }
+  h1, h2, h3, h4, h5, h6 { @apply text-foreground; }
 }
 
 /* Command palette input: remove native borders/shadows to match shadcn */