feat(modal): keyboard-friendly tagging — fandom dialogs, Tab-accept, jump hotkey, cheatsheet

Operator-requested modal/tagging keyboard improvements:
- A2/A3: fandom dialogs autofocus their autocomplete on open; in the character-
  creation FandomPicker, picking a fandom (keyboard Enter or click) confirms in
  one step. FandomSetDialog stays autofocus-only (its Save can trigger a merge).
- B5: Tab accepts the highlighted autocomplete row (standard convention).
- C9: T or / jumps focus to the tag input from anywhere in the modal.
- C8: ? toggles a keyboard cheatsheet (corner hint advertises it; Esc closes the
  cheatsheet first, then the viewer).

Builds on the same-batch regression fixes (kebab #711, ESC-after-accept #700,
autocomplete scroll-into-view). B6 (keep focus after add) is covered — the input
retains focus after adding a tag, and Esc now works after accepting a suggestion.

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

frontend/src/components/modal/ImageViewer.vue

@@ -8,6 +8,25 @@
         <v-icon>mdi-close</v-icon>
       </button>
 
+      <!-- C8: keyboard cheatsheet. '?' toggles; the corner hint advertises it. -->
+      <button
+        class="fc-viewer__help-hint" aria-label="Keyboard shortcuts (press ?)"
+        @click="showHelp = !showHelp"
+      >?</button>
+      <div v-if="showHelp" class="fc-viewer__help" @click.self="showHelp = false">
+        <div class="fc-viewer__help-card" role="dialog" aria-label="Keyboard shortcuts">
+          <h3>Keyboard shortcuts</h3>
+          <dl>
+            <div><dt>← / →</dt><dd>Previous / next image</dd></div>
+            <div><dt>T or /</dt><dd>Jump to the tag input</dd></div>
+            <div><dt>↑ / ↓</dt><dd>Move through tag suggestions</dd></div>
+            <div><dt>Enter / Tab</dt><dd>Accept the highlighted tag</dd></div>
+            <div><dt>Esc</dt><dd>Close a dialog, then the viewer</dd></div>
+            <div><dt>?</dt><dd>Toggle this help</dd></div>
+          </dl>
+        </div>
+      </div>
+
       <v-chip
         v-if="integrityBadge"
         :color="integrityBadge.color"
@@ -63,7 +82,7 @@
 <script setup>
 import { computed, onMounted, onUnmounted, ref, watch } from 'vue'
 import { useModalStore } from '../../stores/modal.js'
-import { arrowNavAllowed } from '../../utils/textEntry.js'
+import { arrowNavAllowed, isTextEntry } from '../../utils/textEntry.js'
 import ImageCanvas from './ImageCanvas.vue'
 import VideoCanvas from './VideoCanvas.vue'
 import TagPanel from './TagPanel.vue'
@@ -74,6 +93,7 @@ const emit = defineEmits(['close'])
 
 const modal = useModalStore()
 const rootEl = ref(null)
+const showHelp = ref(false)
 
 const isVideo = computed(() =>
   modal.current?.mime && modal.current.mime.startsWith('video/')
@@ -96,6 +116,12 @@ let prevBodyOverflow = null
 // that. Filtered via isTextEntry so tag/comment inputs still get their
 // own keystrokes.
 function onKeyDown(ev) {
+  if (ev.key === 'Escape' && showHelp.value) {
+    // Close the cheatsheet first; a second Esc closes the modal.
+    ev.preventDefault()
+    showHelp.value = false
+    return
+  }
   if (ev.key === 'Escape') {
     // Escape closes the modal even from inside a text input — the universal
     // "get me out of here" expectation; the autofocused tag field would
@@ -123,6 +149,14 @@ function onKeyDown(ev) {
     if (!arrowNavAllowed(ev.target)) return
     ev.preventDefault()
     modal.goNext()
+  } else if ((ev.key === '/' || ev.key === 't') && !isTextEntry(ev.target)) {
+    // C9 (2026-06-07): jump focus to the tag input from anywhere in the modal.
+    const input = document.querySelector('.fc-tag-autocomplete input')
+    if (input) { ev.preventDefault(); input.focus() }
+  } else if (ev.key === '?' && !isTextEntry(ev.target)) {
+    // C8: toggle the keyboard cheatsheet.
+    ev.preventDefault()
+    showHelp.value = !showHelp.value
   }
 }
 
@@ -178,6 +212,43 @@ function nextFrame() {
 }
 .fc-viewer__nav:disabled { opacity: 0.3; cursor: not-allowed; }
 .fc-viewer__close { top: 16px; right: 16px; transform: none; }
+.fc-viewer__help-hint {
+  position: absolute; top: 16px; right: 64px; z-index: 2;
+  width: 32px; height: 32px; border-radius: 50%;
+  background: rgba(255, 255, 255, 0.08);
+  color: rgb(var(--v-theme-on-surface));
+  font-weight: 700; cursor: pointer; opacity: 0.6;
+}
+.fc-viewer__help-hint:hover { opacity: 1; }
+.fc-viewer__help {
+  position: absolute; inset: 0; z-index: 4;
+  display: flex; align-items: center; justify-content: center;
+  background: rgba(0, 0, 0, 0.4);
+}
+.fc-viewer__help-card {
+  background: rgb(var(--v-theme-surface));
+  border: 1px solid rgb(var(--v-theme-surface-light));
+  border-radius: 10px; padding: 20px 24px; min-width: 320px;
+  box-shadow: 0 8px 32px rgba(0, 0, 0, 0.5);
+}
+.fc-viewer__help-card h3 {
+  font-family: 'Fraunces', Georgia, serif;
+  font-size: 18px; margin-bottom: 12px;
+  color: rgb(var(--v-theme-on-surface));
+}
+.fc-viewer__help-card dl { display: flex; flex-direction: column; gap: 8px; }
+.fc-viewer__help-card dl > div {
+  display: flex; gap: 16px; align-items: baseline;
+}
+.fc-viewer__help-card dt {
+  flex: 0 0 96px; text-align: right;
+  font-family: 'JetBrains Mono', monospace; font-size: 12px;
+  color: rgb(var(--v-theme-accent));
+}
+.fc-viewer__help-card dd {
+  flex: 1; font-size: 14px;
+  color: rgb(var(--v-theme-on-surface));
+}
 .fc-viewer__integrity {
   position: absolute; top: 72px; right: 16px; z-index: 3;
 }