feat(tui): add status bar, help overlay, tag filter, and entity actions

Status bar with entity count and context-sensitive key hints. Help
overlay via ? key. Tag filter via # with cursor-navigable tag list.
Todo toggle (x), pin (!), promote (p), demote (D), copy (c), edit (e)
via $EDITOR. Delete confirmation with 3s timeout. Date-grouped list
with completed todo and pinned indicators. Esc clears active tag filter.

Adds CompletedAt/ClearCompleted to EntityUpdate for todo toggling.

internal/tui/confirm.go

@@ -0,0 +1,23 @@
+package tui
+
+import (
+	"fmt"
+	"time"
+
+	tea "github.com/charmbracelet/bubbletea"
+
+	"github.com/lerko/nib/internal/display"
+)
+
+type confirmTimeoutMsg struct{}
+
+func confirmTimeout() tea.Cmd {
+	return tea.Tick(3*time.Second, func(time.Time) tea.Msg {
+		return confirmTimeoutMsg{}
+	})
+}
+
+func renderConfirm(entityID string) string {
+	short := display.FormatID(entityID)
+	return errorStyle.Render(fmt.Sprintf("delete %s? y to confirm, any key to cancel", short))
+}