feat: configurable sensitivity and FPS from web UI

[johnoct]

Summary

• Tap the sensitivity pill in the status bar to cycle through Low → Medium → High
• Tap the FPS pill to cycle through 5 → 10 → 15 → 30
• Changes apply immediately to the detection loop and MJPEG stream
• POST /settings endpoint accepts {"sensitivity": "low"|"medium"|"high", "fps": 5|10|15|30}
• State synced from /status on every poll for multi-tab consistency
• Invalid values are silently ignored (server keeps current value)

Test plan

• 178 tests passing (10 new: 4 FrameBuffer + 6 web)
• Verify sensitivity cycling updates detection threshold in real-time
• Verify FPS cycling changes stream smoothness immediately
• Verify settings persist across page refreshes (synced from server)
• Verify invalid POST values are rejected gracefully