fix(lumo): use the 2.0 chat/completions generation endpoint

[vejeta]

Problem

lumo serve and lumo chat fail on every generation:

sse.request url=https://lumo.proton.me/api/ai/v1/chat appversion=web-lumo@2.0.2.9
sse.error  url=https://lumo.proton.me/api/ai/v1/chat status=404 code=404 message="Path not found"
Error: lumo: chat request: api: 404/404: Path not found

Proton retired the legacy PHP generation path ai/v1/chat (404 "Path not
found"). Login, masterkeys, spaces and conversations (all under lumo/v1)
still work and the account is eligible -- only generation moved. Through
lumo serve the 404 surfaces to clients as a generic 500 "Internal server
error".

Fix

Migrate Client.Generate to the Lumo 2.0 unified endpoint
ai/v1/chat/completions, which uses an OpenAI chat/completions-shaped
protocol:

• Request: a messages array (role + encrypted content) plus a lumo
  extension carrying the PGP-wrapped request_key and request_id,
  stream: true, stream_options.include_usage, and reasoning_effort.
• Response: a stream of choices[].delta chunks ending in [DONE],
  parsed back into the existing GenerationResponseMessage types
  (token_data / done / harmful / error).

The AES-GCM content and request-key encryption is reused unchanged; only the
request packaging and the SSE parsing differ. Terminal conditions map to the
existing sentinels (content_filter -> ErrHarmful; an error chunk or a
stream that ends without [DONE] -> ErrStreamClosed).

Verification

• go test ./api/lumo/... passes (the Generate tests are rewritten for the
  new request/response format).
• Live against the real API: lumo chat returns a reply, and the lumo serve
  OpenAI-compatible endpoint returns a completion
  (/v1/chat/completions -> {"choices":[{"message":{"content":"Hello"}...}]}).
• Confirmed the fix is independent of the app-version: generation succeeds with
  the current web-lumo@1.3.3.4 in service_config.go, so no version bump is
  bundled here.

Notes

• StreamProcessor / the legacy generation_request types are left in place
  (still exercised by their unit tests); they can be pruned separately.
• Server-side tool-call/tool-result and image chunks are recognized and skipped
  (this client does not surface them).