Fix API ban error handling and add session token auth

[pkhamchuai]

Problem

The MangaPlus API now requires a Session-Token header. Without it, the
server returns an "Account Banned" protobuf error in response field 2 —
undocumented and outside the generated schema — so it was silently dropped,
causing a confusing IndexError on viewer.pages[-1].

Changes (mloader/loader.py)

• Session token auth — reads Session-Token from token.txt at startup
  so the token can be renewed without touching source code
• _parse_api_error() — manually decodes the failure protobuf field and
  surfaces a clear RuntimeError (e.g. "Account Banned: ...") instead of
  crashing with an IndexError
• Empty viewer guard — raises immediately with a meaningful message when
  the API returns an empty viewer response

Test plan

• Place a valid session token in token.txt and confirm chapters download
• Use an invalid/missing token and confirm a clear RuntimeError is raised
  instead of IndexError

[RhinoDance]

works like a charm! thanks a lot!