feat(timeline): focus follows archive, snaps to first on view load

- New selection.focusAfter(excluded) walks selection.order forward past
  the archived/restored set so X-ing through the timeline keeps the
  cursor on the next live photo instead of falling back to photo[0]
  via the auto-anchor effect. Wired into gridKeyNav.toggleArchive (X
  key) and BulkActionBar.onArchive.
- Auto-focus effect on the timeline always re-anchors to photos[0] on
  view load (pageCount → 1), instead of preserving a stale uid from
  the previous filter.
- PhotoGrid re-anchors focus when the previously focused uid isn't in
  the new photo set, so drilling into a /tags category drops the
  cursor on its first tile instead of carrying a stale selection from
  whatever view the user came from.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/lib/stores/selection.svelte.ts

@@ -113,3 +113,51 @@ export function setFocused(uid: string | null): void {
 export function setAnchor(uid: string | null): void {
 	selection.anchor = uid;
 }
+
+/**
+ * Advance focus to the photo immediately after `excluded` in the current
+ * order, skipping any uid that's in `excluded`. Falls back to the closest
+ * non-excluded uid *before* the excluded set when the user is already at
+ * the tail. Returns `null` when nothing else is left.
+ *
+ * Used right after a mutation that removes the focused photo from the
+ * current view (archive / restore / approve / delete) — calling this
+ * *before* the photo cache refetches keeps focus stable instead of the
+ * effect-driven anchor falling back to photo[0].
+ */
+export function focusAfter(excluded: Iterable<string>): string | null {
+	const excludedSet = excluded instanceof Set ? excluded : new Set(excluded);
+	const order = selection.order;
+	if (order.length === 0) {
+		setFocused(null);
+		return null;
+	}
+	// Anchor index: prefer current focus, else the first excluded uid we
+	// can find (covers the case where focus was already null).
+	let anchorIdx = indexOf(selection.focused);
+	if (anchorIdx === -1) {
+		for (let i = 0; i < order.length; i++) {
+			if (excludedSet.has(order[i])) {
+				anchorIdx = i;
+				break;
+			}
+		}
+	}
+	if (anchorIdx === -1) return null;
+	for (let i = anchorIdx + 1; i < order.length; i++) {
+		if (!excludedSet.has(order[i])) {
+			setFocused(order[i]);
+			setAnchor(order[i]);
+			return order[i];
+		}
+	}
+	for (let i = anchorIdx - 1; i >= 0; i--) {
+		if (!excludedSet.has(order[i])) {
+			setFocused(order[i]);
+			setAnchor(order[i]);
+			return order[i];
+		}
+	}
+	setFocused(null);
+	return null;
+}