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/FandomSetDialog.vue

@@ -8,6 +8,7 @@
           :items="store.fandomCache"
           :item-title="(f) => f.name" :item-value="(f) => f.id"
           label="Fandom" clearable density="compact"
+          autofocus
           :hint="selectedId == null
             ? 'No fandom — the character will be unassigned.' : ''"
           persistent-hint