Add resilient focus session planning

[MohammadSyed18]

What changed

• checkpoint active sessions and restore them after app restart
• reconcile running timers against wall-clock time after suspension
• shorten recommendations when recent sessions are interruption-heavy

Why

Session history was durable, but an in-progress session could be lost when the app backgrounded or restarted. Recovery now preserves the focus plan and adapts it to interruption patterns.

Validation

• npm test (8 tests)
• git diff --check