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/db/entities.go

@@ -71,6 +71,7 @@ type ListParams struct {
 	From           *string
 	To             *string
 	Since          *time.Time
+	ModifiedBefore *time.Time
 	CardsOnly      bool
 	IncludeDeleted bool
 	CardTypeFilter *CardType
@@ -216,6 +217,10 @@ func listWhere(params ListParams) (string, []any) {
 		where = append(where, "e.card_type = ?")
 		args = append(args, string(*params.CardTypeFilter))
 	}
+	if params.ModifiedBefore != nil {
+		where = append(where, "e.modified_at < ?")
+		args = append(args, params.ModifiedBefore.Format(time.RFC3339))
+	}
 
 	clause := ""
 	if len(where) > 0 {
@@ -239,6 +244,8 @@ func (s *Store) List(params ListParams) ([]*Entity, error) {
 	switch params.Sort {
 	case "use_count":
 		orderCol = "e.use_count"
+	case "modified_at":
+		orderCol = "e.modified_at"
 	case "created_at", "":
 		orderCol = "e.created_at"
 	default: