Shell: single-instance tile entries, and a multiselect admin field

Two small pieces of contract for modules that offer several tiles from live
data:

- A `tiles()` entry may say `single: true`: at most one such tile per
  dashboard. The ＋ picker greys the entry out once it is placed ("Already on
  this dashboard") and `addTile` refuses a second — for a control surface
  that would only confuse in duplicate, a send box say.
- Admin config gains `multiselect`: several picks from one list, the same
  `options` / `optionsRoute` as `select`, one switch per option, stored as an
  array of values. A saved value the list no longer offers stays checked and
  marked rather than being dropped in silence. The wrapper is a <label>, so
  it is told to label nothing — otherwise a click anywhere would flip the
  first switch.

Both documented in the module guide.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

public/admin/admin.css

@@ -156,6 +156,12 @@ body { overflow: auto; }
 .endpoint-row .ep-host { flex: 1; min-width: 0; }
 .endpoint-row .ep-port { width: 92px; flex: none; }
 .endpoint-row .ep-sep { color: var(--muted); font-weight: 600; }
+/* several picks from a list (manifest type `multiselect`) */
+.field.multiselect-field { cursor: default; }
+.field.multiselect-field .multiselect { display: flex; flex-direction: column; gap: 6px; }
+.multiselect-row { display: flex; align-items: center; gap: 8px; cursor: pointer; }
+.multiselect-empty { color: var(--muted); font-size: 12px; }
+
 .module-form-empty { color: var(--muted); font-size: 12px; margin-top: 10px; }
 
 /* labeled config subsections (manifest field `group`) */