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/templates/admin/_layout.html

@@ -47,6 +47,11 @@
           {{ icon("sparkle", "icon--sm") }}
           <span class="nav-item__label">Prompts</span>
         </a>
+        <a class="nav-item {{ 'is-active' if section == 'suggestions' }}"
+           href="/admin/suggestions">
+          {{ icon("star", "icon--sm") }}
+          <span class="nav-item__label">Suggestions</span>
+        </a>
         <a class="nav-item {{ 'is-active' if section == 'users' }}" href="/admin/users">
           {{ icon("user", "icon--sm") }}
           <span class="nav-item__label">Users</span>