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/stores/labels.ts

@@ -5,6 +5,7 @@ import { api } from "../api/client";
 export interface Label {
   id: string;
   name: string;
+  color: string;
 }
 
 export const useLabelsStore = defineStore("labels", () => {
@@ -37,10 +38,16 @@ export const useLabelsStore = defineStore("labels", () => {
     sort();
   }
 
+  async function setColor(id: string, color: string): Promise<void> {
+    const updated = await api.patch<Label>(`/api/labels/${id}`, { color });
+    const idx = items.value.findIndex((lb) => lb.id === id);
+    if (idx >= 0) items.value[idx] = updated;
+  }
+
   async function remove(id: string): Promise<void> {
     await api.del(`/api/labels/${id}`);
     items.value = items.value.filter((lb) => lb.id !== id);
   }
 
-  return { items, loaded, load, create, rename, remove };
+  return { items, loaded, load, create, rename, setColor, remove };
 });