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/ui-new/PopupMenu.h

@@ -17,16 +17,14 @@ struct PopupMenu {
   int         _cap;       // actual visible cap, updated each render()
   bool        active;
   const char* _title;
-  bool        anchor_tr = false;  // opt-in: drop from the top-right ≡ hint instead of centring
 
   enum Result { NONE, SELECTED, CANCELLED };
 
   PopupMenu() : _count(0), _sel(0), _scroll(0), _visible(3), _cap(3), active(false), _title(nullptr) {}
 
-  void begin(const char* title, int visible = 3, bool anchor_top_right = false) {
+  void begin(const char* title, int visible = 3) {
     _count = 0; _sel = 0; _scroll = 0;
     _visible = visible; _cap = visible; active = true; _title = title;
-    anchor_tr = anchor_top_right;
   }
 
   void addItem(const char* item) {
@@ -88,17 +86,8 @@ struct PopupMenu {
     int bh = title_h + vis * item_h + 1;   // +1 keeps the last row off the bottom border
 
     // Centre on screen (clamped to the margins on small displays).
-    int bx, by;
-    if (anchor_tr) {
-      // Drop out of the top-right ≡ hint: right-aligned under the header, so the
-      // menu reads as emerging from the glyph that spawned it.
-      bx = display.width() - bw - margin;
-      by = display.headerH();
-      if (by + bh > display.height() - margin) by = display.height() - margin - bh;
-    } else {
-      bx = (display.width()  - bw) / 2;
-      by = (display.height() - bh) / 2;
-    }
+    int bx = (display.width()  - bw) / 2;
+    int by = (display.height() - bh) / 2;
     if (bx < margin) bx = margin;
     if (by < margin) by = margin;