Fix the settings tabs, and space a form from what follows it

**The Audio tab rendered nothing.** The tabs are radios plus sibling
selectors, and the CSS named every tab twice -- once to highlight its label,
once to show its panel. A tab added without also adding those two rules gets a
label that selects nothing, which is not something anyone catches in review; it
looks like a blank page.

Replaced with rules that derive what they can. The active label is
`input:checked + .tabs__tab`, which needs to know nothing at all. The panel is
matched by position -- CSS cannot compare a radio's id with a panel's data-tab
-- so the Nth radio shows the Nth panel. Both lists render in the same order
and a conditional tab drops out of both at once, so they cannot drift. There is
a test asserting the two orders match, including with Audio absent.

**A card following a form sat flush against Save.** The "Try it" panel on the
search page read as another field of the settings form. The gap belongs to the
form rather than to its action row: the action row is always its form's last
child, so a bottom margin there has nothing to push away from. Adds
`.form-actions` and a bottom margin on a form that is a direct child of an
admin page.

Also says plainly in the dictation settings that a server hosting one model
ignores the model field, so `whisper-1` there is a label rather than a
selection.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lembas/web/templates/admin/audio.html

@@ -61,8 +61,9 @@
         <input class="input" id="stt-model" name="stt_model"
                value="{{ values.stt_model }}" placeholder="whisper-1">
         <p class="field__hint">
-          Sent even to servers that only host one; a router in front of several
-          needs it.
+          A server hosting one model <strong>ignores this</strong> and uses
+          whatever it was started with — <code>whisper-1</code> is then just a
+          label. It only selects anything on a server that hosts several.
         </p>
       </div>
       <div class="field">
@@ -179,6 +180,6 @@
     <div id="audio-test-tts"></div>
   </section>
 
-  <div class="btn-row"><button class="btn btn--primary" type="submit">Save settings</button></div>
+  <div class="form-actions"><button class="btn btn--primary" type="submit">Save settings</button></div>
 </form>
 {% endblock %}

src/lembas/web/templates/admin/general.html

@@ -84,6 +84,6 @@
     </p>
   </section>
 
-  <div class="btn-row"><button class="btn btn--primary" type="submit">Save settings</button></div>
+  <div class="form-actions"><button class="btn btn--primary" type="submit">Save settings</button></div>
 </form>
 {% endblock %}

src/lembas/web/templates/admin/search.html

@@ -128,7 +128,7 @@
     </div>
   </section>
 
-  <div class="btn-row"><button class="btn btn--primary" type="submit">Save settings</button></div>
+  <div class="form-actions"><button class="btn btn--primary" type="submit">Save settings</button></div>
 </form>
 
 <section class="card">