Fix timeline poll not removing deleted recordings (#331)

The pollForNewRecordings function had two bugs:
1. It returned early when polledSegments was empty, so deleting all
   recordings in a time range would never clear the timeline.
2. It only detected additions (addedSegs) and never removals, so any
   deleted recording would remain visible until a full page reload.

Fix: compute removedSegs by diffing current IDs against polled IDs.
When either additions or removals are detected, replace the displayed
segments with the authoritative polled list (sorted). Show a toast
notification for removals in the same style as the existing additions
notification.

Author: matteius

web/js/components/preact/timeline/TimelinePage.jsx

@@ -944,22 +944,34 @@ export function TimelinePage() {
         if (!response.ok) return;
         const data = await response.json();
         const polledSegments = data.segments || [];
-        if (polledSegments.length === 0) return;
 
         const currentSegs = timelineState.timelineSegments || [];
         const currentIds = new Set(currentSegs.map(s => String(s.id)));
+        const polledIds = new Set(polledSegments.map(s => String(s.id)));
         const addedSegs = polledSegments.filter(s => !currentIds.has(String(s.id)));
+        const removedSegs = currentSegs.filter(s => !polledIds.has(String(s.id)));
+
+        if (addedSegs.length === 0 && removedSegs.length === 0) return;
+
+        // Use polled segments as the authoritative list
+        const updated = [...polledSegments].sort((a, b) => a.start_timestamp - b.start_timestamp);
+        setSegments(updated);
+        timelineState.setState({ timelineSegments: updated });
 
         if (addedSegs.length > 0) {
-          const merged = [...currentSegs, ...addedSegs].sort((a, b) => a.start_timestamp - b.start_timestamp);
-          setSegments(merged);
-          timelineState.setState({ timelineSegments: merged });
           showStatusMessage(
             `${addedSegs.length} new recording${addedSegs.length !== 1 ? 's' : ''} added to timeline`,
             'info',
             3000
           );
         }
+        if (removedSegs.length > 0) {
+          showStatusMessage(
+            `${removedSegs.length} recording${removedSegs.length !== 1 ? 's' : ''} removed from timeline`,
+            'info',
+            3000
+          );
+        }
       } catch (err) {
         console.debug('Timeline poll error (suppressed):', err);
       }