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.

src/helpers/ui/SSD1306Display.h

@@ -34,6 +34,9 @@ public:
 
   int getCharWidth()  const override { return 6 * _text_sz; }
   int getLineHeight() const override { return 8 * _text_sz; }
+  // Classic Adafruit_GFX built-in font pads every measured string by one
+  // trailing advance column (see DisplayDriver::textWidthTrailingGap()).
+  int textWidthTrailingGap() const override { return 1; }
 
   bool isOn() override { return _isOn; }
   void turnOn() override;