fix(models): register Gemma-4-31B-it in Model enum + MODEL_CONFIG

[aistackdev]

Summary

PR #1 added `gemma4: 'Gemma-4-31B-it'` to `DB_MODEL_TO_DISPLAY` in `packages/constants`, but missed the parallel registration in the app's own `Model` enum + `MODEL_CONFIG` in `packages/app/src/lib/data-mappings.ts`.

Net effect: `MODEL_OPTIONS` (derived from `MODEL_CONFIG` keys) doesn't include Gemma-4-31B-it. Then in `GlobalFilterContext`:

```ts
const availableModels = useMemo(() => MODEL_OPTIONS.filter((m) => ...), [...]);
```

iterates over `MODEL_OPTIONS` (no gemma4 entry), so `availableModels = []` even though `availabilityRows` has gemma4 data. PR #4's fallback effect checks `if (availableModels.length === 0) return;` and bails. User is left stuck on `Model.DeepSeek_R1` (the hardcoded default, has no data) → "No data for DeepSeek-R1-0528" empty state.

Fix

Add to `data-mappings.ts`:

```ts
enum Model {
...

• Gemma_4_31B = 'Gemma-4-31B-it',
  }

const MODEL_CONFIG = {
...

• [Model.Gemma_4_31B]: { label: 'Gemma-4 31B', prefix: 'gemma4', category: 'default' },
  };
  ```

Test plan

• `pnpm typecheck` clean
• Post-deploy: hard-refresh `/evaluation` — should auto-select Gemma-4-31B-it and show 12 rows.

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