refactor(core): remove store global singleton, thread store explicitly

Remove store.Get()/SetGlobal()/Current. Store is now passed explicitly
to all consumers via constructor parameters and function arguments.

- TUI Model holds store field, set via InitialModel(isAdmin, store)
- monitor.StartEngine(s) and InitHistoryFromStore(s) accept store
- server.Start(cfg, s) closes over store in HTTP handlers
- main.go threads store to SSH server, TUI, monitor, server
- isKeyAllowed receives store as parameter

No more hidden dependency on package-level mutable state in store pkg.
Monitor package still uses package-level state (LiveState, etc.) — will
be encapsulated into Engine struct in Phase 7.

internal/store/store.go

@@ -35,13 +35,3 @@ type Store interface {
 	ExportData() (models.Backup, error)
 	ImportData(data models.Backup) error
 }
-
-var Current Store
-
-func SetGlobal(s Store) {
-	Current = s
-}
-
-func Get() Store {
-	return Current
-}