fix: open Project chats on stock Hermes gateways

[rusty4444]

Summary

Fixes #100 with the stock Hermes gateway contract.

Project chats currently require projects.assign_session, but stock Hermes does not ship that sibling RPC. The app therefore blocks the chat before it can open. The existing PR #101 identifies the right cwd-based fallback, but its session.create request still includes the unsupported session_id field and also contains an unrelated feature commit.

Changes

• treat only a missing projects.assign_session sibling as a non-fatal compatibility fallback
• carry the selected Project's primary working directory into the chat
• create the new gateway session with cwd and no client-supplied session_id, matching stock Hermes SessionCreateParams
• retain the runtime session ID returned by the gateway in the existing mobile-to-gateway mapping
• keep transport and genuine Project assignment failures blocking and retryable
• add widget, draft-model, and WebSocket request-shape regression coverage

A future gateway that implements projects.assign_session still uses the explicit assignment path. The cwd fallback is only used when that method is absent.

Verification

• flutter analyze --no-pub: no issues
• flutter test --no-pub: 949 passed
• git diff --check

This is a clean, single-scope alternative to #101. The original cwd fallback was proposed by @Thaeland and is credited in the commit.