refactor(companion): shared shutdown steps, SD channel key by secret, fetch race, small UI fixes

- UITaskBase holds what both UIs' shutdown() and low-battery check share:
  the saves before the goodbye sound, the power-down (GPS through its
  provider) and the battery EMA and threshold.
- L2 SD archive: a channel's file is keyed by its secret only, so renaming
  keeps the history; old files are renamed once at boot.
- L2 fetch: abandoning a request and the task finishing it are one critical
  section (it could stay DONE for good); TileCache gets the read-side barrier.
- Text from a repeater or sensor no longer rings on the device (nothing to
  open); a settings switch reads any non-zero value as on; the message
  toast never cuts a letter in half; ui-new's GPS helpers ask UiCore.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

examples/companion_radio/ui-core/SettingsSchema.h

@@ -391,6 +391,7 @@ static void writeRaw(NodePrefs& p, const Setting& s, int32_t v) {
 // (e.g. an auto-off time set on another frontend).
 static uint8_t get(const NodePrefs& p, const Setting& s) {
   int32_t raw = readRaw(p, s);
+  if (!s.values && s.count == 2 && !s.option) return raw != 0;   // a switch: any non-zero is on (advert_loc_policy 2 from the app)
   if (!s.values) return raw >= 0 && raw < s.count ? (uint8_t)raw : 0;
   uint8_t best = 0;
   int32_t best_d = INT32_MAX;

examples/companion_radio/ui-core/UiCore.h

@@ -205,6 +205,9 @@ public:
                        const uint8_t* extra, int extra_len, const char* text) override {
     // we only want to show text messages on display, not cli data
     if (!(txt_type == TXT_TYPE_PLAIN || txt_type == TXT_TYPE_SIGNED_PLAIN)) return;
+    // Only chats and rooms have a conversation on the device: text from a
+    // repeater or sensor would ring with nothing to open (the app still gets it).
+    if (from.type != ADV_TYPE_CHAT && from.type != ADV_TYPE_ROOM) return;
     if (from.type == ADV_TYPE_ROOM && _room_unread < _queue_len) _room_unread++;
     if (from.type == ADV_TYPE_CHAT) dm_unread.onIncoming(from.id.pub_key);   // before the ring insert below
     UiEvent& ev = pushMessageArrived(from.type == ADV_TYPE_ROOM ? UIEventType::roomMessage : UIEventType::contactMessage,