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/views/ExploreView.vue

@@ -22,9 +22,9 @@
         <button
           v-for="(c, i) in store.breadcrumb" :key="c.id"
           class="fc-ex__crumb"
-          :class="{ 'fc-ex__crumb--current': i === store.breadcrumb.length - 1 }"
+          :class="{ 'fc-ex__crumb--current': i === store.cursor }"
           type="button"
-          :aria-current="i === store.breadcrumb.length - 1 ? 'page' : undefined"
+          :aria-current="i === store.cursor ? 'page' : undefined"
           :title="`Step ${i + 1}`"
           @click="goTo(c.id)"
         >
@@ -191,13 +191,30 @@ function goTo (id) {
 
 function openInViewer (id) { modal.open(id) }
 
-// Modal-parity focus: T or "/" jumps to the tag input (the same shortcut the
-// image modal binds), unless the caret is already in a text field.
+// Keyboard: T or "/" jumps to the tag input (modal-parity); ←/→ walk the
+// breadcrumb trail — ← steps back, → goes forward to an already-visited item
+// or, with no forward history, jumps to a random neighbour (keep rabbit-holing).
 function onKeyDown (ev) {
-  if ((ev.key === 't' || ev.key === '/') && !isTextEntry(ev.target)) {
-    const input = document.querySelector('.fc-tag-autocomplete input')
-    if (input) { ev.preventDefault(); input.focus() }
+  if (ev.metaKey || ev.ctrlKey || ev.altKey) return
+  const inText = isTextEntry(ev.target)
+  if (ev.key === 't' || ev.key === '/') {
+    if (!inText) {
+      const input = document.querySelector('.fc-tag-autocomplete input')
+      if (input) { ev.preventDefault(); input.focus() }
+    }
+    return
   }
+  if (ev.key !== 'ArrowLeft' && ev.key !== 'ArrowRight') return
+  // The tag input auto-focuses after every walk, so also allow navigation while
+  // it's focused-but-EMPTY (the caret has nowhere to go) — otherwise arrow-nav
+  // would dead-end after one step. Once a tag is being typed, arrows move the
+  // caret instead.
+  const t = ev.target
+  const inEmptyTagInput =
+    inText && t.value === '' && t.closest?.('.fc-tag-autocomplete')
+  if (inText && !inEmptyTagInput) return
+  const id = ev.key === 'ArrowLeft' ? store.backTarget() : store.forwardTarget()
+  if (id != null) { ev.preventDefault(); goTo(id) }
 }
 onMounted(() => document.addEventListener('keydown', onKeyDown))
 onUnmounted(() => document.removeEventListener('keydown', onKeyDown))