fix: Increase default httpx timeout and expose timeout parameter (#202)

[bvanelli]

Summary

• The httpx client in ActualServer was created without an explicit timeout, so httpx's default 5s applied to every request — including /sync/sync. This caused intermittent httpcore.ReadTimeout errors on actual.commit() for users on slower servers or larger payloads (httpcore.ReadTimeout: The read operation timed out on actual.commit() #202).
• The Node.js Actual client imposes no timeout at all on the binary sync endpoint, so 5s is materially stricter than upstream.
• This PR raises the default to 60s and exposes a timeout constructor parameter on both ActualServer and Actual. Accepts a float, None (disabled), or an httpx.Timeout object for fine-grained control.

Closes #202.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.62%. Comparing base (5680e04) to head (9fe6834).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #203   +/-   ##
=======================================
  Coverage   97.62%   97.62%           
=======================================
  Files          21       21           
  Lines        3237     3238    +1     
=======================================
+ Hits         3160     3161    +1     
  Misses         77       77           

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