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/DisplayDriver.h

@@ -172,11 +172,21 @@ public:
     else if (count >= 10) { buf[0]=(char)('0'+count/10); buf[1]=(char)('0'+count%10); buf[2]=0; }
     else                  { buf[0]=(char)('0'+count); buf[1]=0; }
   }
+  // Trailing blank column baked into a backend's own advance-based
+  // getTextWidth() (0 if it's already ink-tight). Adafruit_GFX's classic
+  // built-in font measures every character as a fixed 6px advance cell (5px
+  // glyph + 1px inter-character gap) regardless of the glyph actually drawn —
+  // so getTextWidth() always reports 1px more than the real ink, all of it
+  // trailing the last character. Centring on the raw width then leaves 1px
+  // more slack on the right than the left. Overridden by backends that use
+  // that font.
+  virtual int textWidthTrailingGap() const { return 0; }
   // Pixel width the pill from drawUnreadBadge(count) occupies — for reserving
   // the name column before it. Mirrors the pill's horizontal padding.
   int unreadBadgeWidth(int count) {
     char buf[5]; fmtBadgeCount(buf, count);
-    return getTextWidth(buf) + (sepH() + 1) * 2;
+    int pad = sepH() + 1;
+    return getTextWidth(buf) - textWidthTrailingGap() + pad * 2;
   }
   // Right-aligned unread-count "pill": a filled capsule ending at right_x,
   // aligned to a text row of height getLineHeight() at y, with the count
@@ -186,7 +196,8 @@ public:
   // Restores ink to LIGHT. Returns the pill width.
   int drawUnreadBadge(int right_x, int y, int count, bool sel) {
     char buf[5]; fmtBadgeCount(buf, count);
-    int pw = getTextWidth(buf) + (sepH() + 1) * 2;
+    int pad = sepH() + 1;
+    int pw = getTextWidth(buf) - textWidthTrailingGap() + pad * 2;
     int ph = getLineHeight();
     int px = right_x - pw;
     Color body = sel ? DARK : LIGHT;
@@ -198,7 +209,7 @@ public:
     fillRect(px + pw - 1, y, 1, 1);
     fillRect(px, y + ph - 1, 1, 1);
     fillRect(px + pw - 1, y + ph - 1, 1, 1);
-    setCursor(px + sepH() + 1, y);
+    setCursor(px + pad, y);
     print(buf);
     setColor(LIGHT);
     return pw;
@@ -250,7 +261,20 @@ public:
   // menu_open highlights it while the context menu is on screen.
   void drawCenteredHeader(const char* title, bool menu_hint = false, bool menu_open = false) {
     setColor(LIGHT);
-    drawTextCentered(width() / 2, 0, title);
+    int reserve = menu_hint ? menuHintWidth() : 0;
+    char buf[96];
+    translateUTF8ToBlocks(buf, (title && title[0]) ? title : "", sizeof(buf));
+    int avail = width() - reserve - 4;
+    if (avail < 0) avail = 0;
+    if (getTextWidth(buf) <= avail) {
+      int w = getTextWidth(buf);
+      setCursor((width() - reserve) / 2 - w / 2, 0);   // centred, clear of the ≡ hint
+      print(buf);
+    } else {
+      // Too wide to centre without print() wrapping onto a second line → left-align
+      // and ellipsize, like a list row. (Long DM / room-server / channel names.)
+      drawTextEllipsized(2, 0, avail, buf);
+    }
     fillRect(0, headerH() - sepH(), width(), sepH());
     if (menu_hint) drawContextMenuHint(LIGHT, menu_open);
   }

src/helpers/ui/SH1106Display.h

@@ -59,6 +59,10 @@ public:
   }
   int getCharWidth() const override { return (_use_lemon ? 5 : 6) * _text_sz; }
   int getLineHeight() const override { return (_use_lemon ? 9 : 8) * _text_sz; }
+  // Only the built-in classic font pads every measured string by one trailing
+  // advance column (see DisplayDriver::textWidthTrailingGap()); the lemon
+  // font's width comes from its own glyph table (ink-tight, no padding).
+  int textWidthTrailingGap() const override { return _use_lemon ? 0 : 1; }
   void setLemonFont(bool enabled) override { _use_lemon = enabled; _vw_dirty = true; }
   bool isLemonFont() const override { return _use_lemon; }
   void translateUTF8ToBlocks(char* dest, const char* src, size_t dest_size) override;

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;