docs: add sent mail detection pattern to DEVELOPMENT.md

[moltboie]

Changes

Documents the address-based sent mail detection pattern introduced in PR #199.

What's documented

• How sent vs received mail is determined (address matching, not boolean flag)
• Self-email handling (one row, both views)
• Key functions involved (getMailHeaders, getAccountStats)
• Note about IMAP layer still using legacy sent column

Context

PR #199 was a significant architectural change — shifting from a sent boolean column to address-based filtering. This affects how new features should query mail data and is important for anyone working on the IMAP layer.

E2E Testing

Documentation-only change. Verified DEVELOPMENT.md renders correctly.

[moltboie]

Self-Review

Discussion thread status:

• New PR, no prior feedback. Docs-only change for DEVELOPMENT.md.

Checked:

• Content: Documents the address-based sent mail detection pattern from PR refactor: detect sent mail by from_address instead of sent flag #199. Covers the key functions (getMailHeaders, getAccountStats), self-email edge case (single row, both views), and note about IMAP layer still using legacy sent column.
• Accuracy: Pattern is consistent with how sent mail detection was implemented in refactor: detect sent mail by from_address instead of sent flag #199.
• Completeness: Notes the IMAP/REST layer discrepancy — important for future contributors.
• CI: ✅ passing.

Issues found:

• None.

Confidence: High