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/web/src/stores/spaceStore.ts

@@ -367,6 +367,7 @@ export const useSpaceStore = create<SpaceState>((set, get) => ({
       name: s.name,
       icon: s.icon,
       banner: s.banner ?? null,
+      avatarColor: s.avatarColor ?? null,
       ownerId: s.ownerId,
       inviteCode: s.inviteCode,
       visibility: s.visibility ?? 'private' as const,
@@ -488,6 +489,7 @@ export const useSpaceStore = create<SpaceState>((set, get) => ({
       name: space.name,
       icon: space.icon,
       banner: space.banner ?? null,
+      avatarColor: space.avatarColor ?? null,
       ownerId: space.ownerId,
       inviteCode: space.inviteCode,
       visibility: space.visibility,