fix: prevent delete-generation failures on 204 responses

[cursor]

Summary

• Fix a critical API-client correctness bug where successful 204 No Content responses were treated as JSON and threw parse errors.
• This bug broke deletion flows (deleteGeneration) despite backend success, causing major user-facing breakage in gallery delete actions.
• Add a focused regression test to ensure 204 delete responses resolve successfully.

Root cause

PixelmuseClient.request() unconditionally called res.json() for all successful responses. The API documents DELETE /api/v1/images/{id} returning 204, which has no body.

Fix

• In src/core/client.ts, return undefined for 204/205 before JSON parsing.
• Add src/core/client.test.ts validating that deleteGeneration() succeeds when fetch returns 204.

Validation

• pnpm build
• pnpm test (all tests passing, including new regression test)