A pin on the title bar, and no more notch

Pointing at a tile's top edge already brings its bar back, which left the
notch doing half a job: it could still hide a bar, but hiding one tile at a
time stopped being the interesting question once the whole dashboard could
fold its bars at once. It is gone.

In its place, a pin beside each tile's label. Pinned, that tile keeps its
bar while auto-hide is on — for the one or two tiles whose controls are
wanted at a glance. It sits next to the label rather than out with the gear
and ✕ because it is about the bar, not about the tile's contents, and it
stays faint until the tile is pointed at or the pin is on, so a wall of
unpinned tiles is not a wall of pins.

Fullscreen keeps its own rule: every bar folds, pinned or not, because that
is a mode for watching. What it loses with the notch it gets back better —
the same hover at a tile's top edge peeks a bar there too, and now as an
overlay, so peeking no longer pushes the tile's content down and disturbs
the auto-fit. The peek timer the notch needed goes with it.

Per-tile `headHidden` gives way to `headPinned` in the layout, on disk and
on the server.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/MODULE-GUIDE.md

@@ -367,9 +367,10 @@ fallback), never a "Text size" field in `instanceSchema`; a timestamps or
 newest-first toggle in the header means no such switch in the gear. The
 gear is for display choices that have no button of their own.
 
-One more thing to know: the user can hide the whole title bar with the
-small notch on its bottom edge (a clean-view mode for wall displays). Your
-controls disappear with it, so nothing essential — status, reconnects,
+One more thing to know: the whole title bar folds away when the dashboard
+auto-hides it, and always in fullscreen (a clean-view mode for wall
+displays) — it comes back only while the pointer is at the tile's top edge.
+Your controls fold away with it, so nothing essential — status, reconnects,
 live data — may exist *only* as a header control.
 
 `sse(path, handlers)` handlers: