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/monitor/history.go

@@ -20,11 +20,7 @@ var (
 	historyMu sync.RWMutex
 )
 
-func InitHistoryFromStore() {
-	s := store.Get()
-	if s == nil {
-		return
-	}
+func InitHistoryFromStore(s store.Store) {
 	all, err := s.LoadAllHistory(maxHistoryLen)
 	if err != nil {
 		AddLog("Failed to load check history: " + err.Error())
@@ -74,8 +70,8 @@ func RecordCheck(siteID int, latency time.Duration, isUp bool) {
 		h.Statuses = h.Statuses[len(h.Statuses)-maxHistoryLen:]
 	}
 
-	if s := store.Get(); s != nil {
-		go func() { _ = s.SaveCheck(siteID, latency.Nanoseconds(), isUp) }()
+	if db != nil {
+		go func() { _ = db.SaveCheck(siteID, latency.Nanoseconds(), isUp) }()
 	}
 }