fix: keep API failures readable when a response is not JSON

[nvtoan0201-swe]

What changed

MuseApi.request and createSession parsed the body with response.json() before checking response.ok. Any failure that is not JSON — Hono's default plain-text 404, a reverse-proxy HTML error page, or an empty body — surfaced in the UI as SyntaxError: Unexpected token ... instead of the real failure.

Both paths now use a shared readApiPayload reader (apps/mobile/src/api-response.ts) that:

• keeps a JSON error message when the server sends one;
• falls back to a readable Request failed (<status>) (or the caller's fallback, e.g. "Could not open your workspace.") when the body is not JSON or carries no message;
• reports a success response with no readable body clearly.

Regression coverage is in apps/mobile/test/api-response.test.ts (plain-text failure, HTML failure, JSON failure, caller fallback, empty 204, non-JSON success body).

Verification

• pnpm test — 210 pass, 0 fail (4 new tests)
• pnpm lint
• pnpm typecheck (root + mobile)
• pnpm --dir apps/worker typecheck

The change is limited to response parsing and was verified with Response fixtures. No simulator/device run.

Integration limits

No provider, platform export or container path is exercised by this change. Runtime behavior is unchanged except for the error text shown when a response is not JSON.

AI assistance was used to prepare this change; I reviewed the code, tests and checks above.