MeshCore: device-control + firmware management API

[M0LTE]

Parent: #137. Radio settings and firmware must be DAPPS-controllable, not hand-provisioned.

• Device-control API: region presets (localisation), TX power (region-capped), channel management (name + 16-byte PSK) — these map 1:1 to companion opcodes already implemented in the PoC (SET_RADIO_PARAMS 0x0B, SET_RADIO_TX_POWER 0x0C, SET_CHANNEL 0x20, GET_CHANNEL 0x1F, SET_ADVERT_NAME 0x08, APP_START→SELF_INFO). Persist in SystemOptions, apply on startup/change.
• Firmware flash/upgrade wrapping esptool: detect board, fetch+verify the per-board merged bin (sha256), release the serial port, flash @ 0x0, verify, restart. Deliberate operator action, not auto-update.

Radio liveness watchdog + reset/recovery is tracked separately. See README "Device-control & firmware API".

[M0LTE]

Device-control + settings UI landed in #164 (5b06389): a MeshCore panel on the Settings page exposes all DAPPS_MESHCORE_* options (region presets, TX power, channel/PSK, budgets, reliability), plus a live status line and a Reset-radio button (GET /MeshCore/status, POST /MeshCore/reset). Region presets / TX power / channel management are all operator-editable now; the watchdog reset is exposed. Still open in this issue: firmware flash/upgrade from DAPPS (wrapping esptool). Renaming scope accordingly.