Make manual pin permanent instead of a 5-minute timeout

"Show X now" now persists indefinitely (survives restarts too, it's in
config.json) instead of auto-expiring back to rotation after 5 minutes.
Implemented by reusing the existing rotation.enabled flag rather than a
separate scheduler-side timer: pinning a screen sets rotation.enabled=False
and records which screen in a new pinned_screen config field. Re-enabling
rotation -- either the "Resume automatic" button or just checking
"Enabled" again on the Rotation settings page -- clears the pin and
resumes normal cycling either way.

Navidrome priority is unchanged: still checked first, still preempts a
pin the instant something's playing, same as it already preempted
rotation.

Verified end-to-end: pin persists across many scheduler ticks (previously
would have reverted after 5 min), both resume paths (button and settings
checkbox) clear it correctly.

web/templates/index.html

@@ -22,9 +22,9 @@
       <span class="badge {{ 'on' if rotation.enabled else 'off' }}">{{ "on" if rotation.enabled else "off" }}</span>
       {{ rotation.screens | join(" → ") }} (every {{ rotation.interval_minutes }} min)
     </dd>
-    {% if status.manual_screen %}
+    {% if status.pinned_screen %}
     <dt>Manual pin</dt>
-    <dd><span class="badge on">{{ screens[status.manual_screen].display_name }}</span> until {{ status.manual_until }}</dd>
+    <dd><span class="badge on">{{ screens[status.pinned_screen].display_name }}</span> -- fixed until resumed</dd>
     {% endif %}
   </dl>
 </div>
@@ -40,7 +40,7 @@
       <button class="secondary" type="submit">Show {{ screen.display_name }}</button>
     </form>
     {% endfor %}
-    {% if status.manual_screen %}
+    {% if status.pinned_screen %}
     <form class="inline" method="post" action="{{ url_for('resume_rotation') }}">
       <button class="secondary" type="submit">Resume automatic</button>
     </form>