MedieInfo U2.12 ReleaseSlim

This version no longer migrates configuration from the old standalone U1.1
Preferences namespaces.

It preserves the current ProjectUNION configuration because the existing
`project-union` namespace and the `miCfg` record are still used unchanged.

Compile first and compare with U2.11:
- Program bytes
- RAM bytes

Then test:
1. Existing WiFi and MQTT configuration
2. SD folders and JPG display/upload
3. Slide editing
4. MQTT plain payload and JSON field
5. Image, slide and mixed playlists
6. OTA
