labels: per-label color

Give labels a color (migration 0011 adds labels.color, server_default
'default' so existing labels keep the neutral chip). The PATCH endpoint now
updates name and/or color; note serialization carries each label's color.
Frontend: a swatch picker per label in the Edit-labels modal, colored chips
on cards + in the editor (LABEL_CHIP_CLASSES), and a color dot on each
sidebar label. Reuses the note color vocabulary. (Graph node coloring rides
this in the graph-liveliness task.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/components/NoteCard.vue

@@ -1,7 +1,7 @@
 <script setup lang="ts">
 import { ref, watch } from "vue";
 import { useNotesStore } from "../stores/notes";
-import { NOTE_CARD_CLASSES, type NoteColor } from "../notes/colors";
+import { LABEL_CHIP_CLASSES, NOTE_CARD_CLASSES, type NoteColor } from "../notes/colors";
 import type { Note } from "../stores/notes";
 import Icon from "./Icon.vue";
 import LinkedText from "./LinkedText.vue";
@@ -28,6 +28,10 @@ watch(
 function cardClass(color: NoteColor): string {
   return NOTE_CARD_CLASSES[color] ?? NOTE_CARD_CLASSES.default;
 }
+
+function labelChip(color: string): string {
+  return LABEL_CHIP_CLASSES[color as NoteColor] ?? LABEL_CHIP_CLASSES.default;
+}
 </script>
 
 <template>
@@ -86,7 +90,8 @@ function cardClass(color: NoteColor): string {
       <span
         v-for="lb in note.labels"
         :key="lb.id"
-        class="rounded-full bg-black/5 px-2 py-0.5 text-xs text-neutral-600 dark:bg-white/10 dark:text-neutral-300"
+        class="rounded-full px-2 py-0.5 text-xs"
+        :class="labelChip(lb.color)"
         >{{ lb.name }}</span
       >
     </div>