test: add hydration-level coverage for onRehydrateStorage (#129)

[chidii]

Summary

Investigated #129 and couldn't reproduce the described bug. Zustand v4's
persist middleware documents onRehydrateStorage as:

(state) => (state?, error?) => void

The outer function runs once pre-hydration; its return value is invoked
again post-hydration with the hydrated state. That's exactly the shape
currently implemented in prefsStore.ts, and it does run — it's not dead
code.

Reference: https://zustand.docs.pmnd.rs/reference/middlewares/persist

What this PR does

Adds hydration-level test coverage that exercises the actual registered
onRehydrateStorage callback (via persist.getOptions()), rather than
re-simulating it. No production code changes.

Covers the acceptance criteria from #129:

• A stored notificationPrefs missing PROOF_TIMEOUT results in
  PROOF_TIMEOUT: true after rehydration.
• A stored notificationPrefs with REWARD_CONFIRMED: false preserves
  that value (not overwritten by defaults).
• An empty stored-prefs object falls back to full defaults.
• The error path (postRehydrationCallback(undefined, error)) is a no-op.

All four pass against the current, unmodified prefsStore.ts.

Why not the originally-proposed fix

The issue proposes collapsing onRehydrateStorage to a single-argument
(state) => {...} form. That would actually break rehydration: Zustand
would call it once with the pre-hydration state and never invoke it
again after the persisted data loads, so mergeNotificationDefaults would
stop running entirely — a regression, not a fix.

Out of scope

• Adding new notification types
• Changes to quietHours or scheduledNotificationIds persistence
  Closes [Bug] prefsStore.onRehydrateStorage callback is dead code — new notification types added to the codebase are never enabled by default for existing users #129

[cybermax4200]

Fix the ci checks

[chidii]

@cybermax4200 ci fixed, please merge