feat(document): improve proxy status polling resilience

[joshtrichards]

• Resolves: # (various, possibly)
• Target version: main

Summary

• Replace the unbounded 100ms polling loop for proxy.php status checks with limited exponential retries.
• Split server-starting retries from transport failures so we can show a more specific fallback error when the built-in CODE server cannot be reached at all.

This avoids indefinite waiting and improves startup robustness / user feedback for built-in server deployments.

TODO

• Adjust tests

Notes

Possible future follow-up:

• unify/extract out shared proxy-status logic
  • we have two proxy status implementations: one in src/document.js and one in src/services/collabora.js (and Office.vue indirectly which uses the latter basically).

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Documentation (manuals or wiki) has been updated or is not required