v6.4.3 — Reliable daily timetable and safer backups

Daily timetable and backup reliability improvements.

• Refresh today's assignments at local midnight and immediately when returning to the app.
• Validate backup contents before restoring, confirm replacement, preserve progress on cancellation and roll back failed local writes.
• Omit account identity fields from portable backups and record the current app version.
• Add a voluntary student testing guide and study-session feedback template.
• Link public test metrics to current CI results.

Validation: clean lint, 124 unit tests, 553 planner checks, production build and six Chromium browser scenarios pass. Coverage includes open-tab midnight rollover, brief screen locks, invalid backup rejection, restore cancellation, saved progress, mobile/desktop guest workflows and cold offline loading. Authenticated cloud restore and every historical backup format have not been verified.

Live app: https://mission-plustwo.web.app/
Validation record: docs/VALIDATION-6.4.3.md