feat(export): add HTML card deck export

Self-contained single-file HTML export for cards. Mobile-first,
dark theme, zero dependencies. Each card type gets its own
interactive treatment: snippet tap-to-copy, template slot filling,
checklist with progress bar, decision structured layout, link
tap targets. Filter chips by type, search across all cards.

Usage: nib export -f html -o deck.html
       nib export -f html -t triage -o triage.html

cmd/export.go

@@ -6,19 +6,28 @@ import (
 	"os"
 
 	"github.com/lerko/nib/internal/db"
+	"github.com/lerko/nib/internal/export"
 	"github.com/spf13/cobra"
 )
 
-var exportOutput string
+var (
+	exportOutput string
+	exportFormat string
+	exportTag    string
+	exportTitle  string
+)
 
 var exportCmd = &cobra.Command{
 	Use:   "export",
-	Short: "dump all entities to JSON",
+	Short: "export entities to JSON or HTML card deck",
 	RunE:  runExport,
 }
 
 func init() {
 	exportCmd.Flags().StringVarP(&exportOutput, "output", "o", "", "write to file instead of stdout")
+	exportCmd.Flags().StringVarP(&exportFormat, "format", "f", "json", "output format: json or html")
+	exportCmd.Flags().StringVarP(&exportTag, "tag", "t", "", "filter by tag (used as deck name for HTML)")
+	exportCmd.Flags().StringVar(&exportTitle, "title", "", "deck title for HTML export")
 	rootCmd.AddCommand(exportCmd)
 }
 
@@ -48,13 +57,58 @@ func runExport(cmd *cobra.Command, _ []string) error {
 	}
 	defer store.Close()
 
-	ctx := cmd.Context()
-
 	p := db.DefaultListParams()
-	p.IncludeDeleted = true
 	p.Limit = 10000
 
-	entities, err := store.List(ctx, p)
+	if exportTag != "" {
+		p.Tag = &exportTag
+	}
+
+	switch exportFormat {
+	case "html":
+		return runHTMLExport(cmd, store, p)
+	case "json":
+		p.IncludeDeleted = true
+		return runJSONExport(cmd, store, p)
+	default:
+		return fmt.Errorf("unknown format %q (use json or html)", exportFormat)
+	}
+}
+
+func runHTMLExport(cmd *cobra.Command, store *db.Store, p db.ListParams) error {
+	p.CardsOnly = true
+
+	entities, err := store.List(cmd.Context(), p)
+	if err != nil {
+		return err
+	}
+
+	title := exportTitle
+	if title == "" && exportTag != "" {
+		title = "#" + exportTag
+	}
+	if title == "" {
+		title = "nib cards"
+	}
+
+	if exportOutput != "" {
+		f, err := os.Create(exportOutput)
+		if err != nil {
+			return err
+		}
+		defer f.Close()
+		if err := export.RenderHTML(f, entities, title); err != nil {
+			return err
+		}
+		fmt.Fprintf(cmd.ErrOrStderr(), "exported %d cards to %s\n", len(entities), exportOutput)
+		return nil
+	}
+
+	return export.RenderHTML(cmd.OutOrStdout(), entities, title)
+}
+
+func runJSONExport(cmd *cobra.Command, store *db.Store, p db.ListParams) error {
+	entities, err := store.List(cmd.Context(), p)
 	if err != nil {
 		return err
 	}