Add SSH source support with dialstdio and UI components

- Implement SSH dial via stdio for remote connections
- Add sources API and storage layer for managing connection sources
- Add SourcePanel and SshFields web components for SSH configuration
- Update app structure to support source-based connections
- Update handlers and server for new sources endpoint

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

main.go

@@ -84,7 +84,7 @@ func serve(args []string) error {
 	token := fs.String("token", os.Getenv("DOCKMV_TOKEN"), "require this token on every request; \"auto\" generates one (default: $DOCKMV_TOKEN)")
 	dataDir := fs.String("data-dir", defaultDataDir(), "directory for connections and trusted host keys")
 	pkgDir := fs.String("package-dir", "", "directory for migration packages (default <data-dir>/packages)")
-	dockerHost := fs.String("docker-host", "", "source docker daemon (default: the DOCKER_HOST environment)")
+	dockerHost := fs.String("docker-host", "", "local source docker daemon (default: the DOCKER_HOST environment)")
 	verbose := fs.Bool("v", false, "verbose logging")
 	fs.Usage = func() {
 		fmt.Fprintln(os.Stderr, "Usage: dockmv serve [flags]\n\nFlags:")
@@ -93,6 +93,14 @@ func serve(args []string) error {
 	if err := fs.Parse(args); err != nil {
 		return err
 	}
+	// A --docker-host given on the command line is an instruction for this run,
+	// and takes precedence over the source remembered from the last one.
+	dockerHostSet := false
+	fs.Visit(func(f *flag.Flag) {
+		if f.Name == "docker-host" {
+			dockerHostSet = true
+		}
+	})
 
 	level := slog.LevelInfo
 	if *verbose {
@@ -113,13 +121,14 @@ func serve(args []string) error {
 	}
 
 	cfg := api.Config{
-		Addr:       *addr,
-		Token:      authToken,
-		DataDir:    *dataDir,
-		PackageDir: *pkgDir,
-		DockerHost: *dockerHost,
-		UI:         webui.FS(),
-		Logger:     logger,
+		Addr:          *addr,
+		Token:         authToken,
+		DataDir:       *dataDir,
+		PackageDir:    *pkgDir,
+		DockerHost:    *dockerHost,
+		DockerHostSet: dockerHostSet,
+		UI:            webui.FS(),
+		Logger:        logger,
 	}
 	srv, err := api.New(cfg)
 	if err != nil {