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/styles.go

@@ -28,6 +28,10 @@ var (
 	glyphStyle = lipgloss.NewStyle().
 			Width(2)
 
+	completedGlyphStyle = lipgloss.NewStyle().
+				Width(2).
+				Foreground(dim)
+
 	tagStyle = lipgloss.NewStyle().
 			Foreground(lipgloss.AdaptiveColor{Light: "#43BF6D", Dark: "#73F59F"})
 
@@ -54,4 +58,23 @@ var (
 	errorStyle = lipgloss.NewStyle().
 			Foreground(lipgloss.Color("#FF0000")).
 			PaddingLeft(1)
+
+	dateHeaderStyle = lipgloss.NewStyle().
+			Foreground(dim).
+			PaddingLeft(1)
+
+	pinnedStyle = lipgloss.NewStyle().
+			Foreground(lipgloss.AdaptiveColor{Light: "#D4A017", Dark: "#FFD700"})
+
+	filterPillStyle = lipgloss.NewStyle().
+			Foreground(lipgloss.AdaptiveColor{Light: "#43BF6D", Dark: "#73F59F"}).
+			Bold(true)
+
+	helpKeyStyle = lipgloss.NewStyle().
+			Foreground(highlight).
+			Bold(true).
+			Width(18)
+
+	helpDescStyle = lipgloss.NewStyle().
+			Foreground(dim)
 )