fix: preserve context across SSE reconnects on tab visibility change

Datastar aborts SSE on visibilitychange (tab hidden) and reconnects
when visible. The previous cleanup-on-disconnect destroyed the context
before the client could reconnect. Now SSE disconnect does a soft
teardown (mark disconnected, keep context alive) and reconnect drains
stale patches before resuming. The reaper uses disconnect time instead
of creation time so recently-disconnected contexts aren't prematurely
reaped.

context.go

@@ -39,8 +39,9 @@ type Context struct {
 	subscriptions     []Subscription
 	subsMu            sync.Mutex
 	disposeOnce       sync.Once
-	createdAt         time.Time
-	sseConnected      atomic.Bool
+	createdAt           time.Time
+	sseConnected        atomic.Bool
+	sseDisconnectedAt   atomic.Pointer[time.Time]
 }
 
 // View defines the UI rendered by this context.