feat(explore): ← / → keyboard navigation through the walk

Arrow keys walk the Explore breadcrumb trail: ← steps back, → goes forward to
an already-visited item or — with no forward history — jumps to a RANDOM
neighbour to keep the rabbit-hole going (operator-asked).

The trail gains a cursor (browser back/forward semantics): stepping back no
longer trims the forward branch, so → can return to it; a genuinely new walk
off a back-step truncates the stale branch then appends. The crumb-bar "current"
highlight follows the cursor, not the tip.

Arrows are ignored while typing a tag, but still navigate when the tag input is
focused-but-empty (it auto-focuses after every walk, so otherwise arrow-nav
would dead-end after one step). Modifier-key combos pass through untouched.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

frontend/src/stores/explore.js

@@ -19,6 +19,10 @@ export const useExploreStore = defineStore('explore', () => {
   const anchor = ref(null)       // /api/gallery/image/<id> payload
   const neighbors = ref([])      // [{id, thumbnail_url, ...}]
   const breadcrumb = ref([])     // [{id, thumbnail_url}] walked path
+  // Index of the current anchor within breadcrumb — browser-style back/forward.
+  // The trail keeps its forward branch when you step back (so ← / → can move
+  // through visited items); a NEW walk off a back-step truncates that branch.
+  const cursor = ref(-1)
   const loading = ref(false)
   const error = ref(null)
 
@@ -54,13 +58,39 @@ export const useExploreStore = defineStore('explore', () => {
     }
   }
 
-  // Forward walk appends; navigating to an id already in the trail (a
-  // breadcrumb click, or a loop back) TRIMS to it — so the route stays the
-  // single source of truth and the crumb bar never grows stale branches.
+  // The route is the source of truth; this reconciles the trail + cursor to it.
+  // Revisiting an id already on the path (← / →, a crumb click, or a loop) just
+  // MOVES the cursor there — the forward branch is preserved so → can return to
+  // it. A genuinely new image off a back-step truncates the stale forward branch
+  // (browser semantics), then appends and points the cursor at the new tip.
   function _reconcileTrail (id, thumbnailUrl) {
     const idx = breadcrumb.value.findIndex((c) => c.id === id)
-    if (idx >= 0) breadcrumb.value = breadcrumb.value.slice(0, idx + 1)
-    else breadcrumb.value = [...breadcrumb.value, { id, thumbnail_url: thumbnailUrl }]
+    if (idx >= 0) {
+      cursor.value = idx
+      return
+    }
+    const base = cursor.value < breadcrumb.value.length - 1
+      ? breadcrumb.value.slice(0, cursor.value + 1)
+      : breadcrumb.value
+    breadcrumb.value = [...base, { id, thumbnail_url: thumbnailUrl }]
+    cursor.value = breadcrumb.value.length - 1
+  }
+
+  // ← target: the previous crumb (null at the start of the trail).
+  function backTarget () {
+    return cursor.value > 0 ? breadcrumb.value[cursor.value - 1].id : null
+  }
+
+  // → target: the next already-visited crumb if we'd stepped back, else a
+  // RANDOM neighbour to keep the rabbit-hole going. Null if neither exists.
+  function forwardTarget () {
+    if (cursor.value >= 0 && cursor.value < breadcrumb.value.length - 1) {
+      return breadcrumb.value[cursor.value + 1].id
+    }
+    if (neighbors.value.length) {
+      return neighbors.value[Math.floor(Math.random() * neighbors.value.length)].id
+    }
+    return null
   }
 
   function reset () {
@@ -68,6 +98,7 @@ export const useExploreStore = defineStore('explore', () => {
     anchor.value = null
     neighbors.value = []
     breadcrumb.value = []
+    cursor.value = -1
     error.value = null
     loading.value = false
   }
@@ -145,8 +176,8 @@ export const useExploreStore = defineStore('explore', () => {
   function close () {}
 
   return {
-    anchor, neighbors, breadcrumb, loading, error, NEIGHBOR_LIMIT,
-    anchorOn, reset,
+    anchor, neighbors, breadcrumb, cursor, loading, error, NEIGHBOR_LIMIT,
+    anchorOn, reset, backTarget, forwardTarget,
     // host surface
     current, currentImageId,
     reloadTags, addExistingTag, removeTag, createAndAdd, close,