MicroPython completeness pass: 6 more Python templates + photo-first onboarding

- Python templates added: Servo, OLED, PIR Motion, Ultrasonic, BLE, Joystick — all
  heavily commented for teaching. 12-of-12 Python templates now shipped.
- Welcome modal replaced with a multi-step onboarding flow:
    * Step 1: choose how to identify your board — Photo / QR / Pick from list / Skip
    * Photo path: camera or upload -> Gemini Vision 'identify this board' prompt ->
      confirmation card ('That's right' vs. 'Not quite')
    * QR path: uses browser BarcodeDetector API; graceful fallback if unsupported
    * Manual path: dropdown of common ESP32 variants + generic fallback
    * Done step: summary + 3-step intro card with the sunset-gradient circle motif
- Identified board saves to settings.boardDescription; also syncs to the hidden
  ai-board dropdown when one of the listed variants matches.
- buildUserPrompt now includes 'User's exact hardware setup: <description>' when set,
  so every AI response is tuned to whatever the user identified.
- Camera and QR streams get stopped cleanly on Back/Close/skip to avoid lingering
  permissions indicators.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: CodingWithGrandmaDotCom