feat(tui): add interactive run mode for checklists and fill mode for templates

Run mode (r key on checklist cards): cursor navigates steps, space
toggles done/undone, r resets all, esc saves changes to DB and exits.
Persists step state — improvement over web which discards on exit.

Fill mode (f key on template cards): tab/shift-tab navigates slots,
type to fill values, enter resolves template and copies to clipboard
with use count increment. Esc cancels without copying.

Both modes are sub-states of detail view, keeping architecture simple.

internal/tui/fill.go

@@ -0,0 +1,152 @@
+package tui
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/charmbracelet/bubbles/textinput"
+	tea "github.com/charmbracelet/bubbletea"
+
+	"github.com/lerko/nib/internal/carddata"
+)
+
+type fillSlot struct {
+	Name    string
+	Default string
+	Value   string
+}
+
+type fillModel struct {
+	slots    []fillSlot
+	active   int
+	body     string
+	entityID string
+	ti       textinput.Model
+}
+
+func newFillModel(entityID, body string) fillModel {
+	slots := discoverSlots(body)
+	m := fillModel{
+		slots:    slots,
+		body:     body,
+		entityID: entityID,
+	}
+	m.ti = textinput.New()
+	m.ti.CharLimit = 200
+	if len(slots) > 0 {
+		m.ti.Placeholder = slots[0].Name
+		m.ti.Focus()
+		if slots[0].Default != "" {
+			m.ti.SetValue(slots[0].Default)
+		}
+	}
+	return m
+}
+
+func discoverSlots(body string) []fillSlot {
+	matches := carddata.TemplateSlotRe.FindAllStringSubmatch(body, -1)
+	seen := map[string]bool{}
+	var slots []fillSlot
+	for _, m := range matches {
+		name := m[1]
+		if !seen[name] {
+			seen[name] = true
+			slots = append(slots, fillSlot{Name: name})
+		}
+	}
+	return slots
+}
+
+func (f fillModel) resolve() string {
+	result := f.body
+	for _, s := range f.slots {
+		val := s.Value
+		if val == "" {
+			val = "${" + s.Name + "}"
+		}
+		result = strings.ReplaceAll(result, "${"+s.Name+"}", val)
+	}
+	return result
+}
+
+func (f fillModel) update(msg tea.KeyMsg) (fillModel, tea.Cmd) {
+	switch msg.String() {
+	case "tab":
+		f.commitActive()
+		if f.active < len(f.slots)-1 {
+			f.active++
+		} else {
+			f.active = 0
+		}
+		f.loadActive()
+		return f, nil
+	case "shift+tab":
+		f.commitActive()
+		if f.active > 0 {
+			f.active--
+		} else {
+			f.active = len(f.slots) - 1
+		}
+		f.loadActive()
+		return f, nil
+	}
+
+	f.ti, _ = f.ti.Update(msg)
+	return f, nil
+}
+
+func (f *fillModel) commitActive() {
+	if f.active < len(f.slots) {
+		f.slots[f.active].Value = f.ti.Value()
+	}
+}
+
+func (f *fillModel) loadActive() {
+	if f.active < len(f.slots) {
+		s := f.slots[f.active]
+		f.ti.SetValue(s.Value)
+		f.ti.Placeholder = s.Name
+		f.ti.Focus()
+	}
+}
+
+func (f fillModel) view(width int) string {
+	if len(f.slots) == 0 {
+		return statusStyle.Render("no slots")
+	}
+
+	var b strings.Builder
+	header := fmt.Sprintf("⤓ filling   slot %d/%d", f.active+1, len(f.slots))
+	b.WriteString(detailHeaderStyle.Render(header))
+	b.WriteString("\n\n")
+
+	for i, slot := range f.slots {
+		name := detailLabelStyle.Render(slot.Name)
+		var val string
+		if i == f.active {
+			val = f.ti.View()
+		} else if slot.Value != "" {
+			val = detailValueStyle.Render(slot.Value)
+		} else {
+			val = idStyle.Render("(empty)")
+		}
+
+		if i == f.active {
+			b.WriteString(selectedItemStyle.Render(" " + name + "  " + val))
+		} else {
+			b.WriteString(listItemStyle.Render(name + "  " + val))
+		}
+		b.WriteString("\n")
+	}
+
+	b.WriteString("\n")
+	preview := f.resolve()
+	if len(preview) > width-4 {
+		preview = preview[:width-7] + "…"
+	}
+	b.WriteString(detailBodyStyle.Render(preview))
+	b.WriteString("\n\n")
+	b.WriteString(helpStyle.Render("tab:next  shift+tab:prev  enter:copy  esc:cancel"))
+
+	return b.String()
+}