fix: client perms, @everyone/@channel, docs, unit tests

- usePermissions ORs current user roles + @everyone only (not all server roles)
- cache myRolesByServer; load on active server; refresh after self role edit
- gate/notify @everyone and @channel; plain @username push; special mention UI
- refresh FEATURE_PARITY (DMs exist; drop stale critical gaps)
- README production deploy notes dumpster.service
- unit tests for permission bits and broadcast mention tokens

web/src/stores/permissions.ts

@@ -55,7 +55,8 @@ export const PERMISSION_LABELS: Record<PermissionKey, string> = {
 };
 
 export function hasPermission(perms: number, flag: number): boolean {
-  return (perms & flag) !== 0;
+  // Match backend permissions.Has: all required bits must be present.
+  return (perms & flag) === flag;
 }
 
 export interface ChannelOverride {