fix: Improve mousehole fallback handling and UI feedback

[masonfox]

Summary

Fixes the mousehole fallback UI to properly display when state.json is not found and Scurry falls back to the static token file.

Problem

When MOUSEHOLE_ENABLED=true but the state.json file doesn't exist (e.g., mousehole not running or volume mount incorrect), the UI showed:

• Status: "Waiting for mousehole..."
• Source: secrets/mam_api_token (confusing - shows static file but implies waiting for mousehole)
• No indication that fallback is being used

Solution

API Changes (app/api/mam-token/route.js)

• Added usingFallback: true flag to mouseholeInfo when falling back to static token
• Added descriptive error message: "state.json not found, using fallback token file"
• Ensures fallback response includes proper mousehole metadata

UI Changes (app/components/TokenManager.jsx)

• Added yellow warning banner when usingFallback is true
• Shows clear message: "Mousehole State File Not Found"
• Explains that static fallback token is being used
• Provides troubleshooting hint about volume mounts
• Status now shows "Using fallback token" instead of "Waiting for mousehole..."
• Displays correct source file path regardless of mode

After Fix

The UI now clearly shows:

• ⚠️ Yellow warning banner explaining the fallback state
• Status: "Using fallback token"
• Source: secrets/mam_api_token (accurate)
• Helpful hint to verify mousehole is running and volumes are mounted correctly

Testing

• ✅ Mousehole enabled with valid state.json → Shows purple banner
• ✅ Mousehole enabled without state.json → Shows yellow warning banner
• ✅ Mousehole disabled → Shows normal manual management UI
• ✅ Fallback token works correctly for API calls

Fixes the issue shown in production where the UI was confusing when mousehole wasn't available.