feat(tui): stumble mode — resurface stale entries card by card

Card-by-card walkthrough of entries untouched for 30+ days.
Prevents write-mostly decay by bringing old entries back to attention.

- S from list triggers stumble, loads entries where modified_at < 30d
- Single-card view with markdown body, glyph, tags, age indicator
- Actions: n skip, d dismiss, ! pin, p promote, m absorb, esc exit
- Progress indicator: stumble [3/12]
- After promote/absorb from stumble, returns to deck (not list)
- "All caught up" screen when deck exhausted
- DB: add ModifiedBefore to ListParams, modified_at sort column

internal/tui/styles.go

@@ -143,4 +143,7 @@ var (
 
 	railCountStyle = lipgloss.NewStyle().
 			Foreground(dim)
+
+	stumbleAgeStyle = lipgloss.NewStyle().
+			Foreground(lipgloss.AdaptiveColor{Light: "#cc4400", Dark: "#fab387"})
 )