Mobile: tour bottom-sheet + measured sizing; tidy profile tab-grid tiles

Onboarding tour (tourEngine.js):
- On narrow viewports (<640px) dock the popover as a fixed BOTTOM SHEET
  instead of anchoring it — kills the off-screen/jump-to-center glitches
  caused by short viewports, wrapping buttons and URL-bar resize. The
  spotlight still highlights the target, scrolled to center so it sits above
  the sheet.
- On desktop, measure the ACTUAL popover size (getBoundingClientRect) and
  feed it to computeTourPlacement, instead of the fixed 240/336 guesses that
  pushed tall popovers off-screen.

Profile modal mobile tab grid (CSS):
- Equal-height tiles via a min-height token + centered content, so long
  labels no longer leave a ragged grid.
- Move the unread dot to the tile's top-right corner (was landing in the
  grid gutter at the vertical-nav left edge).
- Tighter line-height/padding + non-shrinking glyph.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

css/tokens.css

@@ -359,6 +359,8 @@
   --notification-banner-max-inline: var(--size-28rem);
   --profile-modal-nav-min-width: max-content;
   --profile-modal-nav-max-width: max-content;
+  /* Equal-height floor for the mobile tab-grid tiles. */
+  --profile-modal-nav-tile-min-height: 4.75rem;
   --profile-modal-scrollbar-offset: var(--space-2xs);
   --modal-embed-height: 80vh;
   --profile-history-max-height: 26.25rem;