feat(companion): on-device scope + repeater scope filtering; fix CAD, UTF-8 truncation, Public channel, Nodes list, keyboard cursor

- Settings > Radio > Scope: type a community/region name on-device (derives
  the shared key the same "#name" -> SHA256 way as DEFAULT_FLOOD_SCOPE_NAME),
  previously only settable from a connected app.
- Tools > Repeater > Scope only + Extra scopes: only relay flood traffic
  matching the device's own scope or a comma-separated list of additional
  scopes, without changing what scope the device's own messages send under.
  No-op while unconfigured.
- getCADEnabled()/getInterferenceThreshold() were hardcoded off on
  companion_radio; CAD now auto-enables whenever RX power-save (duty-cycle)
  is active, since the noise floor isn't kept fresh during duty-cycle sleep.
- Message truncation to fit the send frame could split a multi-byte UTF-8
  character in half; now stops at the last complete character.
- The default "Public" channel was unconditionally re-added at every boot
  before the saved channel list was loaded, so deleting it never stuck.
  Only seeded now on a genuinely fresh device (no channel file yet).
- Tools > Nodes read contacts from the wrong starting offset, landing on
  internally-reserved bookkeeping slots instead of real contacts -- showed
  as blank "Unknown" rows and silently dropped that many real contacts off
  the end of the list.
- resetContacts() only cleared the first few reserved slots, not the whole
  contact table, contrary to its own comment; only reachable today via
  private-key import, fixed to match stated intent regardless.
- Keyboard's multi-line text preview could render the cursor on an empty
  line below short typed text instead of right after it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/solo_features/settings_screen/settings_screen.md

@@ -72,6 +72,7 @@ Lists all available home screen pages. For each entry:
 | CR        | 5–8        | LEFT/RIGHT. Coding rate (4/5–4/8). |
 | Pwr save  | ON / OFF   | **Battery saver.** Hardware duty-cycle receive (SX126x only): cycles RX↔sleep, wakes on preamble, cuts average RX current at the cost of some latency. **Forced off (`--`) while the repeater is on** — restored once it's switched off. A background watchdog auto-recovers if the sequencer gets stuck (soft re-arm, then a full reset) — see Tools › Diagnostics for the counts. |
 | Auto pwr  | ON / OFF   | **Adaptive Power Control.** Lowers TX power on strong links, ramps back to the **TX Pwr** ceiling on weak/lost ones. Link quality from DM ACK SNR, or — for channels (no ACK) — a repeater's rebroadcast. Live power shown on the radio page/name bar. Default OFF. **Suppressed (`--`) while the repeater is on** — restored once it's switched off. |
+| Scope     | text       | Names your device's radio "community" (e.g. `pl`) — typing a name derives a shared key the same way on every device, so any device that types the same name lands on the same scope automatically, no key exchange needed. Tags this device's own DM/channel sends so repeaters can tell your community's flood traffic apart from others sharing the same frequency; paired with **Tools › Repeater › Scope only**, it's also what this device relays for in repeater mode. Not encryption — message content is unaffected either way. **Enter** opens the keyboard; empty clears it (unscoped, the previous default). |
 
 |           OLED            |           E-Ink           |
 | :-----------------------: | :-----------------------: |
@@ -79,7 +80,7 @@ Lists all available home screen pages. For each entry:
 
 <!-- screenshot pending: Radio — preset popup (pick/save/delete) and/or the digit-by-digit frequency editor -->
 
-The **repeater** mode and its flood filters live on their own screen — see **Tools › Repeater**.
+The **repeater** mode and its flood filters live on their own screen — see **Tools › Repeater**, including how it uses **Scope** above.
 
 ---