fix(p2p): LWW timestamp parsing (replace string comparison)

[mads-jm]

LWW uses string comparison, not timestamp parsing (app/src/renderer/db/replication-handler.ts:60). Works for ISO-8601 by coincidence; brittle to format drift / clock skew. Parse to epoch.

⚠️ Touches the P2P protocol — needs explicit human approval before agentic work (CLAUDE.md).
Plan & context: WhatNext - docs/06 reports/report-260627-issue-reconciliation.md and report-260627-mvp-state-of-the-union.md.