[callumalpass]

Integrates the first round of the sync polish campaign. Each fix comes with a regression test that failed before it; reports and evidence are on each agent branch.

• Rebuild main.js for integration round 1
• Keep conflict copies idempotent and refuse raced copy overwrites
• Migrate the latest mirror checkpoint after browser approval
• Give sync recovery one honest status and next action
• Reject binary downloads whose destination changed while streaming
• Coalesce transfer pane redraws without delaying sync outcomes
• Bound quadratic merge work so large notes cannot freeze Obsidian
• Validate CRLF frontmatter with its terminating newline
• Discard late renewals of retired connections and preserve current sync scope
• Reuse preview consent checks while transfer progress changes
• Honor Stop sync before starting transfers after inspection
• Preserve frontmatter whitespace and moves instead of silently dropping edits
• Respect offline retry backoff instead of preempting it with probes
• Index receipt actions once to avoid quadratic sync history work

Validation: npm test (278 pass, plus 10 lossless round trips), npm run lint, and npm run check:mobile.