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/chat/index.html

@@ -184,6 +184,22 @@
             {{ mark(cls="empty__mark", uid="intro") }}
             <h2 class="empty__title">What would you ask?</h2>
             <p class="empty__text">Speak, friend, and enter.</p>
+
+            {# Only on a chat that does not exist yet. An empty chat someone
+               opened on purpose already has a model and a prompt chosen. #}
+            {% if not chat and suggestions %}
+            <div class="suggestions">
+              {% for suggestion in suggestions %}
+              <button class="suggestion" type="button"
+                      data-suggestion="{{ suggestion.prompt }}">
+                <span class="suggestion__name">{{ suggestion.name }}</span>
+                {% if suggestion.description %}
+                <span class="suggestion__note">{{ suggestion.description }}</span>
+                {% endif %}
+              </button>
+              {% endfor %}
+            </div>
+            {% endif %}
           </div>
           {% endif %}