fix: retain queued learning edits across tab handoff and storage retry

[sarthakagrawal927]

Summary

• Merge pending operations from another tab instead of overwriting them during sequential handoff.
• Preserve adopted operations through a failed storage write; independent regression reproduced their loss before the repair.
• Add SQLite-backed account/receipt/recovery coverage and Feynman grading-unavailability coverage.

Verification

• Independent review: full suite 967 tests, lint/typecheck/docs passed before final repair.
• Final focused suite: 14 tests passed, including red-before/green-after storage failure case; lint/docs passed.

Remaining acceptance

Refs #97; do not auto-close. Truly overlapping localStorage read/merge/write remains non-atomic and can lose a queued operation; documented rather than claimed solved. Remote migration 0003 is already on main but must be verified/applied before a production deployment. No migration or deployment in this PR. Hosted/account/mobile qualification remains open.