* fix(sync): reflect share status from global store after cancel
Fix#1551: unshareSession() called updateLiveSession() which silently
fails when the child store doesn't exist. The sidebar rendered from the
child store first, showing stale share data. Now overlays the global
session's share field at merge points.
* fix(sync): extract shared mergeLiveSessionWithGlobalSession helper
Extracted the share-field overlay into a single shared helper in
useGlobalSessionsStore.ts. All 3 merge sites now use the helper
instead of duplicating the overlay logic.
* test(sync): add unit tests for mergeLiveSessionWithGlobalSession helper
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Co-authored-by: Leonid Skorobogatyy <bash@opencode.itc.local>
