feat: space avatar color with color picker UI

Add avatarColor field to spaces, matching the user avatar color system.
Spaces get a random color on creation and owners can change it in space
settings. The color controls the fallback gradient when no icon is uploaded,
replacing the old deterministic hash-based gradient. Includes full
federation support, explore page, mutual spaces, and color picker in both
create and settings modals.

packages/server/src/routes/explore.ts

@@ -146,6 +146,7 @@ function buildFullSpace(spaceId: string, forUserId: string): SpaceWithChannelsAn
     name: space.name,
     icon: space.icon,
     banner: space.banner ?? null,
+    avatarColor: (space.avatarColor as SpaceWithChannelsAndMembers['avatarColor']) ?? null,
     ownerId: space.ownerId,
     inviteCode: space.inviteCode,
     visibility: (space.visibility ?? 'private') as SpaceWithChannelsAndMembers['visibility'],
@@ -203,7 +204,7 @@ export async function exploreRoutes(app: FastifyInstance): Promise<void> {
 
     let countSql = `SELECT COUNT(DISTINCT s.id) as total FROM spaces s WHERE s.visibility IN ('public', 'request')`;
     let querySql = `
-      SELECT s.id, s.name, s.icon, s.banner, s.description, s.visibility, s.created_at,
+      SELECT s.id, s.name, s.icon, s.banner, s.avatar_color, s.description, s.visibility, s.created_at,
              COUNT(sm.user_id) as member_count
       FROM spaces s
       LEFT JOIN space_members sm ON sm.space_id = s.id
@@ -227,6 +228,7 @@ export async function exploreRoutes(app: FastifyInstance): Promise<void> {
       name: string;
       icon: string | null;
       banner: string | null;
+      avatar_color: string | null;
       description: string | null;
       visibility: string;
       created_at: number;
@@ -238,6 +240,7 @@ export async function exploreRoutes(app: FastifyInstance): Promise<void> {
       name: r.name,
       icon: r.icon,
       banner: r.banner,
+      avatarColor: (r.avatar_color as ExploreSpace['avatarColor']) ?? null,
       description: r.description,
       visibility: r.visibility as ExploreSpace['visibility'],
       memberCount: r.member_count,