fix(tui): track selection by site ID + q means back everywhere

Cursor tracked by site ID instead of positional index. When the
list re-sorts every tick (sites change status), the selection stays
on the same monitor instead of silently jumping to whatever now
occupies that index position.

q now means "back" in detail, history, SLA, and alert-detail views
— consistent with muscle memory from navigating deeper views.
Only the dashboard q quits the app. ctrl+c always quits from
anywhere.

internal/tui/tui.go

@@ -103,6 +103,7 @@ type Model struct {
 	state        sessionState
 	currentTab   int
 	cursor       int
+	selectedID   int
 	tableOffset  int
 	maxTableRows int
 	termWidth    int