Create key-vault.ts with AES-256-GCM encrypted IndexedDB storage for
API keys (Claude, OpenRouter). Add ApiKeyManager.vue settings UI with
masked key display and add/remove functionality. Integrate vault lookups
into useAI.ts streaming functions with graceful fallback when IDB is
unavailable.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>