fix(ui): revert corner-anchored context menus, centre unread badge, add room logout

- Context menus (PopupMenu, Nearby/QuickMsg call sites) go back to centring on
  screen; the header's discoverable-menu glyph stays, but dropping the popup
  out of its corner looked bad on some screens.
- Unread pill badge digit wasn't centred: Adafruit_GFX's classic built-in font
  (SH1106/SSD1306) always pads a measured string by one trailing advance
  column regardless of the glyph drawn, so centring on the raw width left 1px
  more slack on the right than the left. New DisplayDriver::
  textWidthTrailingGap() (0 by default) corrects for it on those two backends.
- On-device room Logout: mirrors the app's CMD_LOGOUT (drops keep-alive
  tracking, forgets the saved password) so a room can be deliberately signed
  out of from the Room options menu, not just re-logged-in.

Not build-verified — no PlatformIO toolchain available this session.

examples/companion_radio/MyMesh.h

@@ -230,6 +230,16 @@ public:
     return true;
   }
 
+  // On-device UI logout: drops the local keep-alive tracking (mirrors the app's
+  // CMD_LOGOUT) and forgets the saved password, so the next room open prompts
+  // for credentials again instead of silently re-using them. No packet is sent
+  // to the server -- the room ACL has no session state to tear down, this is
+  // purely local "forget this login" bookkeeping.
+  void logoutRoom(const uint8_t* pub_key) {
+    stopConnection(pub_key);
+    forgetRoomPassword(pub_key);
+  }
+
   // On-device-saved room/repeater login passwords, persisted to flash (own
   // small file, independent of /contacts3) so a room that's already been
   // logged into doesn't need its password retyped after reboot.