v0.12.2: restore section simplification — snapshot filtering, auto-stop/restart, UI cleanup

- StackDataProvider interface extended with StopStack/StartStack
- backup.Manager.GetStackHDDMounts() delegates to stackProvider
- RestoreApp() auto-stops app before restic restore, restarts after (even on failure)
- stackAdapter in main.go wires StopStack/StartStack through to stacks.Manager
- GET /api/backup/snapshots?stack={name} filters snapshots by app HDD paths via filterSnapshotsByPaths()
- Restore section simplified: no path list, per-app filtered snapshots, human-friendly timestamp format, single calm warning, empty-result inline message

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

controller/cmd/controller/main.go

@@ -407,6 +407,14 @@ func (a *stackAdapter) ListDeployedStacks() []backup.StackSummary {
 	return result
 }
 
+func (a *stackAdapter) StopStack(name string) error {
+	return a.mgr.StopStack(name)
+}
+
+func (a *stackAdapter) StartStack(name string) error {
+	return a.mgr.StartStack(name)
+}
+
 func (a *stackAdapter) GetStackHDDMounts(name string) []string {
 	s, ok := a.mgr.GetStack(name)
 	if !ok {