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/internal/backup/appdata.go

@@ -14,6 +14,8 @@ type StackDataProvider interface {
 	GetStackComposePath(name string) (composePath string, ok bool)
 	ListDeployedStacks() []StackSummary
 	GetStackHDDMounts(name string) []string
+	StopStack(name string) error
+	StartStack(name string) error
 }
 
 // StackSummary holds minimal stack info needed for app data discovery.