feat: support --after/--before date filtering for IMAP sync

[arunim1]

Summary

• Wire --after/--before flags through to IMAP SEARCH SINCE/BEFORE criteria instead of ignoring them
• Only warn about --query (which has no IMAP equivalent); date flags now work natively
• No behavior change when flags are omitted (zero time = no filter)

This makes recurring IMAP syncs practical — scanning a rolling window via --after instead of enumerating the full mailbox each time.

Tested locally: --after with a recent date narrowed the server-side scan dramatically.

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invalid date formats are silently ignored instead of returning an error

IIRC this is how the repo handles invalid date formats more generally (e.g. for Gmail too).

Solves this issue: #195

🤖 Generated with Claude Code

[roborev-ci]

roborev: Combined Review (0a64afa)

Verdict: The changes successfully add IMAP support for date filtering, but introduce a medium-severity issue where invalid date formats are silently ignored instead of returning an error.

Medium

• Location: cmd/msgvault/cmd/syncfull.go:216-222
• Problem: Invalid --after or --before values are silently ignored because time.Parse errors are dropped. A command like sync --after not-a-date will proceed without filtering and unexpectedly sync far more mail than requested instead of warning the user.
• Fix:
  Validate the date flags once, check if err != nil, and return an explicit error (e.g., fmt.Errorf("invalid --after date format (use YYYY-MM-DD): %w", err)) to halt the sync on malformed input. Reuse the parsed values for both
  Gmail query construction and IMAP client setup.

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Synthesized from 3 reviews (agents: codex, gemini | types: default, security)

[roborev-ci]

roborev: Combined Review (0494659)

Verdict: No medium-or-higher findings; the change looks clean based on all review outputs.

All three reviews found no issues, and there were no medium, high, or critical findings to report.

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Synthesized from 3 reviews (agents: codex, gemini | types: default, security)