fix: persist Inbox 'Show unread only' toggle across sessions

[iroiro147]

Problem

The Inbox's Show unread only toggle resets to off whenever the Inbox view is remounted. Users who prefer an unread-only Inbox have to enable it again after navigating away, reopening the Inbox, or restarting Buzz.

Closes #3669

Root Cause

The unreadOnly state in HomeView.tsx was a plain React.useState(false) with no persistence layer — every remount starts fresh.

Solution

Introduced usePersistentBoolean — a small localStorage-backed hook that follows the same versioned-key pattern already used by useFeedItemState in this codebase (e.g. buzz-home-feed-done.v1).

• On mount, reads the stored value from localStorage. If absent, falls back to false.
• On every state change, writes the new value back to localStorage.
• SSR-safe (typeof window guard) and degrades silently if storage is unavailable.

Changes

File	Change
desktop/src/features/home/usePersistentBoolean.ts	New hook (40 lines)
desktop/src/features/home/ui/HomeView.tsx	Replace useState(false) with usePersistentBoolean (2 lines changed)
desktop/src/features/home/usePersistentBoolean.test.mjs	7 tests covering default, restore true/false, toggle, key convention

Acceptance Criteria

• Enabling Show unread only remains effective after navigating away from and back to the Inbox
• The enabled state is restored after restarting Buzz
• Disabling the toggle updates the saved preference so it remains disabled on subsequent visits
• Users without a saved preference retain the current default of off
• Automated coverage verifies preference restoration and the default/fallback behavior

Test Plan

cd desktop
node --test src/features/home/usePersistentBoolean.test.mjs

All 7 tests pass:

• default is false when no stored value exists
• stored true is restored as true
• stored false is restored as false
• writing true then reading returns true
• writing false then reading returns false
• toggling from true to false persists the new value
• storage key uses versioned prefix matching existing buzz-home patterns