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/migrate.ts

@@ -105,6 +105,12 @@ export function runMigrations(db: Database.Database): void {
       columns: [
         { name: 'is_deleted', type: 'INTEGER DEFAULT 0' },
       ]
+    },
+    {
+      name: 'spaces',
+      columns: [
+        { name: 'avatar_color', type: 'TEXT' },
+      ]
     }
   ];