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.

config.py

@@ -42,6 +42,7 @@ class Config:
         data["rotation"].setdefault("interval_minutes", DEFAULT_ROTATION["interval_minutes"])
         data["rotation"].setdefault("enabled", DEFAULT_ROTATION["enabled"])
         data.setdefault("navidrome_priority", True)
+        data.setdefault("pinned_screen", None)
         data.setdefault("settings", {})
         for screen_id, screen in screens.items():
             data["settings"].setdefault(screen_id, {})
@@ -81,6 +82,30 @@ class Config:
     def set_rotation(self, screens: list[str], interval_minutes: int, enabled: bool) -> None:
         with self._lock:
             self._data["rotation"] = {"screens": screens, "interval_minutes": interval_minutes, "enabled": enabled}
+            if enabled:
+                # Re-enabling rotation (e.g. via the settings checkbox)
+                # supersedes any manual pin -- that's the intended way to
+                # get back to automatic switching.
+                self._data["pinned_screen"] = None
+        self.save()
+
+    def get_pinned_screen(self) -> str | None:
+        with self._lock:
+            return self._data.get("pinned_screen")
+
+    def pin_screen(self, screen_id: str) -> None:
+        """Pin a screen indefinitely -- turns rotation off so it doesn't
+        fight with the pin. Stays until the pin is explicitly cleared or
+        rotation is re-enabled (settings checkbox, or resume_rotation())."""
+        with self._lock:
+            self._data["pinned_screen"] = screen_id
+            self._data["rotation"]["enabled"] = False
+        self.save()
+
+    def resume_rotation(self) -> None:
+        with self._lock:
+            self._data["rotation"]["enabled"] = True
+            self._data["pinned_screen"] = None
         self.save()
 
     def get_navidrome_priority(self) -> bool: