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/db/schema.ts

@@ -25,6 +25,7 @@ export const spaces = sqliteTable('spaces', {
   name: text('name').notNull(),
   icon: text('icon'),
   banner: text('banner'),
+  avatarColor: text('avatar_color'),
   ownerId: text('owner_id').notNull().references(() => users.id),
   inviteCode: text('invite_code').unique(),
   visibility: text('visibility').default('private'),