feat(explore): auto-focus the tag input on every image change

The workspace is built for rapid walk-and-tag, but the tag field was only
focused once (TagAutocomplete's on-mount autofocus) — walking to a neighbour
left focus behind, so the operator had to click the field each time
(operator-asked 2026-06-26).

TagPanel now exposes focusTagInput; ExploreView watches the focused image id and
re-focuses the field on seed + every walk via nextTick. Reuses the existing
focus path, so TagAutocomplete's mobile guard (no soft-keyboard pop on touch) is
preserved.

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

frontend/src/components/modal/TagPanel.vue

@@ -86,7 +86,11 @@ async function onNavigate(tag) {
 // Return focus to the tag input after a suggestion is accepted (from the
 // Suggestions panel or the autocomplete dropdown) so the operator can keep
 // typing the next tag without re-clicking the field (operator-asked 2026-06-08).
+// Exposed so a host surface (the Explore workspace) can re-focus the field when
+// it swaps the focused image, without re-clicking. The field's own mobile guard
+// (TagAutocomplete) is preserved.
 function focusTagInput() { tagInputRef.value?.focus?.() }
+defineExpose({ focusTagInput })
 
 async function onRemove(tagId) {
   errorMsg.value = null