fix(tags): allow creating a character with no fandom

Not all characters belong to a fandom (original characters, unsorted).
The create flow forced every new character through FandomPicker, whose
only outcomes were 'Use this fandom' (disabled until one is picked) or
Cancel (which aborts the whole creation) — there was no way to confirm a
character with no fandom.

- FandomPicker: add a 'No fandom' action that emits confirm(null).
- TagAutocomplete.onFandomChosen: pass fandom_id: null when null is
  emitted.

Backend already supported this end to end (Tag.fandom_id nullable, the
CHECK only forbids fandom_id on non-character kinds, tag_service
find_or_create defaults fandom_id=None, API reads body.get). A fandom can
still be assigned later from the chip kebab's 'Set fandom…'.

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

frontend/src/components/modal/FandomPicker.vue

@@ -17,6 +17,10 @@
       </div>
     </v-card-text>
     <v-card-actions>
+      <!-- Not every character belongs to a fandom (original characters,
+           unsorted, etc.). "No fandom" creates the character unassigned;
+           a fandom can still be set later from the chip's kebab menu. -->
+      <v-btn variant="text" @click="onNoFandom">No fandom</v-btn>
       <v-spacer />
       <v-btn @click="$emit('cancel')">Cancel</v-btn>
       <v-btn :disabled="!selectedId" color="primary" rounded="pill" @click="onConfirm">Use this fandom</v-btn>
@@ -45,4 +49,9 @@ function onConfirm() {
   const f = store.fandomCache.find(x => x.id === selectedId.value)
   if (f) emit('confirm', f)
 }
+// Create the character with no fandom. Emits null so the caller knows this
+// was a deliberate "unassigned", not a cancel.
+function onNoFandom() {
+  emit('confirm', null)
+}
 </script>

frontend/src/components/modal/TagAutocomplete.vue

@@ -147,10 +147,14 @@ function onCreate () {
   reset()
 }
 
+// fandom is null when the user picked "No fandom" — characters don't all
+// belong to a fandom. The backend already accepts fandom_id: null for the
+// character kind (tag.kind check + nullable fandom_id), and a fandom can be
+// assigned later from the chip kebab's "Set fandom…".
 function onFandomChosen (fandom) {
   fandomDialog.value = false
   emit('pick-new', {
-    name: pendingNewName, kind: 'character', fandom_id: fandom.id,
+    name: pendingNewName, kind: 'character', fandom_id: fandom ? fandom.id : null,
   })
   pendingNewName = null
   reset()