[Manus provider] Replace PR #476 draft behavior with a clean main-based implementation

[hhh2210]

@steipete @ratulsarna

I found several problems in the old Manus PR #476 provider path, and I already implemented a clean main-compatible fix locally.

Problem

PR #476 had a few issues that make it hard to land safely on current main:

• token source semantics were leaky
• manual / off-mode boundaries were not strict enough
• invalid manual input could fall through to other sources instead of failing fast
• Manus usage was forced into generic credits / extra-usage UI that does not match the actual Manus quota model
• the branch has drifted too far from current main to be a safe merge candidate

What I fixed locally

I rebuilt the Manus provider cleanly on top of current main:

• provider descriptor + implementation + settings wiring
• bounded auth resolution:
  • off stays off
  • manual does not silently fall back
  • browser/cache/env fallback is explicit and ordered
• Manus-specific usage mapping:
  • balance
  • monthly credits
  • daily refresh
• official Manus brand asset integration
• dedicated Manus tests for cookie parsing, fallback behavior, cache behavior, and sparse live payload parsing

Validation

• swift test --filter Manus passed locally
• pnpm check passed
• ./Scripts/compile_and_run.sh passed
• verified against a real live Manus session and the live Manus credits API

I found the problem, and I already fixed it locally as a clean replacement for PR #476 rather than trying to revive the drifted branch.

[hhh2210]

PR #700 is up — CI passed, tests pass (swift test --filter Manus), verified against a live Manus session. Ready for review whenever convenient. @ratulsarna