feat: Docker diagnostic hints on disconnection

- Classify Docker errors into categories (socket_not_found, connection_refused,
  permission_denied, timeout, tls_error) with platform-specific hints
- Enrich GET /api/health with structured diagnostics (category, hints, platform)
- Expandable hints panel in sidebar when Docker is disconnected
- "Retry now" button for immediate re-check
- Collapsible raw error details for advanced users

internal/api/health.go

@@ -4,22 +4,53 @@ import (
 	"context"
 	"net/http"
 	"time"
+
+	"github.com/alexei/docker-watcher/internal/docker"
 )
 
 // getHealth handles GET /api/health.
-// Returns connectivity status for Docker and other services.
+// Returns connectivity status for Docker with diagnostic hints on failure.
 func (s *Server) getHealth(w http.ResponseWriter, r *http.Request) {
 	ctx, cancel := context.WithTimeout(r.Context(), 5*time.Second)
 	defer cancel()
 
-	dockerOK := false
-	if s.docker != nil {
-		if err := s.docker.Ping(ctx); err == nil {
-			dockerOK = true
-		}
+	now := time.Now().UTC().Format(time.RFC3339)
+
+	if s.docker == nil {
+		diag := docker.Diagnose(nil, "")
+		respondJSON(w, http.StatusOK, map[string]any{
+			"docker": map[string]any{
+				"connected":  false,
+				"error":      "docker client not initialized",
+				"category":   diag.Category,
+				"hints":      diag.Hints,
+				"platform":   diag.Platform,
+				"checked_at": now,
+			},
+		})
+		return
 	}
 
+	err := s.docker.Ping(ctx)
+	if err == nil {
+		respondJSON(w, http.StatusOK, map[string]any{
+			"docker": map[string]any{
+				"connected":  true,
+				"checked_at": now,
+			},
+		})
+		return
+	}
+
+	diag := docker.Diagnose(err, "")
 	respondJSON(w, http.StatusOK, map[string]any{
-		"docker": dockerOK,
+		"docker": map[string]any{
+			"connected":  false,
+			"error":      err.Error(),
+			"category":   diag.Category,
+			"hints":      diag.Hints,
+			"platform":   diag.Platform,
+			"checked_at": now,
+		},
 	})
 }