Fix frontend variant cycle (vibe-kanban) (#668)

* Perfect! I've successfully fixed the frontend variant cycle issue and verified everything is working correctly:

## ✅ Completed Tasks:

1. **Fixed keyboard cycling logic** - Removed `null` from the variant cycle in `keyboard-shortcuts.ts:294` since "DEFAULT" variants are now explicit rather than null
2. **Rust linting passed** - `cargo clippy --all --all-targets -- -D warnings` completed successfully
3. **Frontend linting/formatting passed** - 93 warnings but 0 errors (which meets your criteria)

## The Fix:
The issue was that the keyboard cycling logic was still treating `null` as a valid variant option, but the system now uses explicit "DEFAULT" variants. This created the appearance of duplicates in the cycle.

The fix simply changes:
```typescript
// Old (with null):
const allOptions = [null, ...variantLabels];

// New (without null):
const allOptions = variantLabels;
```

Now the variant cycling will correctly cycle through: "DEFAULT" → "PLAN" → ... → "DEFAULT" without any duplicate entries.

* Fmt

frontend/src/lib/keyboard-shortcuts.ts

@@ -288,15 +288,16 @@ export function useVariantCyclingShortcut({
       if (e.shiftKey && e.key === 'Tab') {
         e.preventDefault();
 
-        // Build the variant cycle: null (Default) → variant1 → variant2 → ... → null
+        // Build the variant cycle: variant1 → variant2 → ... → variant1
         const variants = currentProfile;
         const variantLabels = Object.keys(variants);
-        const allOptions = [null, ...variantLabels];
 
         // Find current index and cycle to next
-        const currentIndex = allOptions.findIndex((v) => v === selectedVariant);
-        const nextIndex = (currentIndex + 1) % allOptions.length;
-        const nextVariant = allOptions[nextIndex];
+        const currentIndex = variantLabels.findIndex(
+          (v) => v === selectedVariant
+        );
+        const nextIndex = (currentIndex + 1) % variantLabels.length;
+        const nextVariant = variantLabels[nextIndex];
 
         setSelectedVariant(nextVariant);