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

internal/export/html.go

@@ -0,0 +1,181 @@
+package export
+
+import (
+	_ "embed"
+	"encoding/json"
+	"html/template"
+	"io"
+	"strings"
+	"time"
+
+	"github.com/lerko/nib/internal/db"
+	"github.com/lerko/nib/internal/display"
+)
+
+//go:embed template.html
+var templateHTML string
+
+type TemplateSlot struct {
+	Name    string `json:"name"`
+	Default string `json:"default"`
+}
+
+type CheckStep struct {
+	Text string `json:"text"`
+	Done bool   `json:"done"`
+}
+
+type DecisionData struct {
+	Chose    string   `json:"chose"`
+	Why      string   `json:"why"`
+	Rejected []string `json:"rejected"`
+}
+
+type CardView struct {
+	ID           string
+	Glyph        string
+	CardType     string
+	Title        string
+	Body         template.HTML
+	Description  template.HTML
+	SearchText   string
+	Pinned       bool
+	Tags         []string
+	UseCount     int
+	LinkURL      string
+	Slots        []TemplateSlot
+	TemplateBody string
+	Steps        []CheckStep
+	Progress     int
+	Decision     DecisionData
+}
+
+type DeckView struct {
+	Title      string
+	Count      int
+	ExportedAt string
+	Types      []string
+	Cards      []CardView
+}
+
+func RenderHTML(w io.Writer, entities []*db.Entity, title string) error {
+	tmpl, err := template.New("deck").Parse(templateHTML)
+	if err != nil {
+		return err
+	}
+
+	seen := map[string]bool{}
+	var types []string
+	var cards []CardView
+
+	for _, e := range entities {
+		if e.CardType == nil {
+			continue
+		}
+		ct := string(*e.CardType)
+		if !seen[ct] {
+			seen[ct] = true
+			types = append(types, ct)
+		}
+		cards = append(cards, buildCardView(e))
+	}
+
+	data := DeckView{
+		Title:      title,
+		Count:      len(cards),
+		ExportedAt: time.Now().Format("Jan 2, 2006"),
+		Types:      types,
+		Cards:      cards,
+	}
+
+	return tmpl.Execute(w, data)
+}
+
+func buildCardView(e *db.Entity) CardView {
+	ct := ""
+	if e.CardType != nil {
+		ct = string(*e.CardType)
+	}
+
+	title := ""
+	if e.Title != nil {
+		title = *e.Title
+	}
+
+	body := e.Body
+	desc := ""
+	if e.Description != nil {
+		desc = *e.Description
+	}
+
+	search := strings.ToLower(title + " " + body + " " + desc + " " + strings.Join(e.Tags, " "))
+
+	cv := CardView{
+		ID:         e.ID,
+		Glyph:      display.DisplayGlyph(e.Glyph, e.CardType),
+		CardType:   ct,
+		Title:      title,
+		Body:       template.HTML(template.HTMLEscapeString(body)),
+		SearchText: search,
+		Pinned:     e.Pinned,
+		Tags:       e.Tags,
+		UseCount:   e.UseCount,
+	}
+
+	if desc != "" {
+		cv.Description = template.HTML(template.HTMLEscapeString(desc))
+	}
+
+	if e.CardData != nil {
+		parseCardData(&cv, ct, *e.CardData, body)
+	}
+
+	return cv
+}
+
+func parseCardData(cv *CardView, ct string, raw string, body string) {
+	switch ct {
+	case "snippet":
+		// body is the snippet content, already set
+
+	case "template":
+		var data struct {
+			Slots []TemplateSlot `json:"slots"`
+		}
+		if json.Unmarshal([]byte(raw), &data) == nil {
+			cv.Slots = data.Slots
+		}
+		cv.TemplateBody = body
+
+	case "checklist":
+		var data struct {
+			Steps []CheckStep `json:"steps"`
+		}
+		if json.Unmarshal([]byte(raw), &data) == nil {
+			cv.Steps = data.Steps
+			done := 0
+			for _, s := range cv.Steps {
+				if s.Done {
+					done++
+				}
+			}
+			if len(cv.Steps) > 0 {
+				cv.Progress = (done * 100) / len(cv.Steps)
+			}
+		}
+
+	case "decision":
+		var dec DecisionData
+		if json.Unmarshal([]byte(raw), &dec) == nil {
+			cv.Decision = dec
+		}
+
+	case "link":
+		var data struct {
+			URL string `json:"url"`
+		}
+		if json.Unmarshal([]byte(raw), &data) == nil {
+			cv.LinkURL = data.URL
+		}
+	}
+}