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/shared/src/types.ts

@@ -43,6 +43,7 @@ export interface Space {
   name: string;
   icon: string | null;
   banner: string | null;
+  avatarColor: AvatarColor | null;
   ownerId: string;
   inviteCode: string | null;
   visibility: SpaceVisibility;
@@ -55,6 +56,7 @@ export interface ExploreSpace {
   name: string;
   icon: string | null;
   banner: string | null;
+  avatarColor: AvatarColor | null;
   description: string | null;
   visibility: SpaceVisibility;
   memberCount: number;
@@ -323,6 +325,7 @@ export interface CreateSpaceRequest {
   name: string;
   icon?: string;
   banner?: string;
+  avatarColor?: string;
   visibility?: SpaceVisibility;
   description?: string;
 }
@@ -343,6 +346,7 @@ export interface UpdateSpaceRequest {
   name?: string;
   icon?: string;
   banner?: string;
+  avatarColor?: string;
   visibility?: SpaceVisibility;
   description?: string;
 }