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/components/modals/UserProfileModal.tsx

@@ -433,7 +433,7 @@ export function UserProfileModal() {
                         ) : (
                           <div
                             className="w-8 h-8 rounded-lg flex items-center justify-center text-[13px] font-semibold text-white"
-                            style={{ background: getSpaceGradient(space.id, space.name).gradient }}
+                            style={{ background: getSpaceGradient(space.id, space.name, space.avatarColor).gradient }}
                           >
                             {space.name.charAt(0).toUpperCase()}
                           </div>