fix(errors): set this.name in every domain error constructor (R6)

Spec reviewer caught a systemic gap during Task 6 (navigation) review:
Plan 8 left every feature's domain error class with a constructor that
didn't set this.name = '<ClassName>'. Plan 9 spec R6 requires it.

Fixed across all 10 error files:
- auth: AuthenticationError, UnauthenticatedError, UnauthorizedError, InputParseError
- blog: ArticleNotFoundError, InputParseError
- marketing-pages: PageNotFoundError, InputParseError
- navigation: HeaderNotFoundError, InputParseError
- media: MediaNotFoundError, InputParseError

Functionally a no-op — defineErrorMiddleware uses instanceof, not name —
but ensures correct serialization, stack traces, and JSON inspection.

Refactor log: §7
Spec: R6

packages/auth/src/entities/errors/auth.ts

@@ -1,17 +1,20 @@
 export class AuthenticationError extends Error {
   constructor(message: string, options?: ErrorOptions) {
     super(message, options);
+    this.name = "AuthenticationError";
   }
 }
 
 export class UnauthenticatedError extends Error {
   constructor(message: string, options?: ErrorOptions) {
     super(message, options);
+    this.name = "UnauthenticatedError";
   }
 }
 
 export class UnauthorizedError extends Error {
   constructor(message: string, options?: ErrorOptions) {
     super(message, options);
+    this.name = "UnauthorizedError";
   }
 }

packages/auth/src/entities/errors/common.ts

@@ -1,5 +1,6 @@
 export class InputParseError extends Error {
   constructor(message: string, options?: ErrorOptions) {
     super(message, options);
+    this.name = "InputParseError";
   }
 }