fix(gmail-browser): include thread identity in fallback ids

[WaldenLee2005]

Summary

• include Gmail thread identity in messageId-less fallback row ids so same-content rows in different threads no longer collide
• bump the Gmail browser seen key version to 3 so existing seen stores rebaseline after the fallback id shape changes
• add tests covering thread-aware fallback ids and the seen key version bump

Tests

• cargo fmt -p puffer-cli
• git diff --check
• cargo test -p puffer-cli gmail_browser

Manual verification

• configured gmail-browser against waikn31@gmail.com
• confirmed normal live Gmail polling emitted both same-subject test messages
• temporarily forced fallback ids locally and observed distinct fallback dedup keys:
  • waikn31@gmail.com:cbf5e0a83
  • waikn31@gmail.com:c7e7bc727
• removed the temporary debug edit before commit

Fixes #772.

[milhous]

Reviewed locally. Scope is limited to gmail-browser fallback row identity and seen key migration; CI is green and local checks passed: cargo fmt -p puffer-cli --check, git diff --check, cargo test -p puffer-cli gmail_browser.