Prompt suggestions on the new-chat screen

A blank composer is the least helpful thing a chat client can show someone
who has just installed one. Three cards now sit under the empty state, and
an administrator manages them at /admin/suggestions.

Clicking a card fills the composer and stops there. It deliberately does not
send: every default ends mid-sentence, because a card is a starting point
rather than a question somebody already asked, and the caret lands where the
person has to start typing.

Seeding is guarded by a settings flag, not by "is the table empty" --
otherwise an administrator who decided against them would get all three back
on every restart. Capped at twelve, six shown: past a dozen this is a menu,
and a menu on the empty screen is a worse blank page than a blank page.

The cards are gated on there being no chat at all, not on the thread being
empty. An empty chat someone opened on purpose already has a model and a
prompt chosen.

Also fixes a pre-existing bug the position test caught. Both this and
_refresh_models wrote `coalesce(max(position), -1) or -1`, and position 0 is
falsy -- so the second row landed back on 0 on top of the first. The
coalesce was already doing that job; the `or` was undoing it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lembas/web/static/js/app.js

@@ -384,6 +384,21 @@
       return;
     }
 
+    /* A suggestion card fills the composer and stops there. It deliberately
+       does not submit: the prompts end mid-sentence, because a card is a
+       starting point rather than a question somebody already asked. */
+    var suggestion = event.target.closest("[data-suggestion]");
+    if (suggestion) {
+      event.preventDefault();
+      var input = document.querySelector("[data-composer-input]");
+      if (!input) return;
+      input.value = suggestion.dataset.suggestion;
+      autosize(input);
+      input.focus();
+      input.setSelectionRange(input.value.length, input.value.length);
+      return;
+    }
+
     /* Show/hide a panel by selector, so templates do not each carry their own
        inline toggle script. */
     var toggle = event.target.closest("[data-toggle]");