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/keys.go

@@ -33,6 +33,7 @@ type keyMap struct {
 	FocusRight key.Binding
 	Tab        key.Binding
 	ToggleRail key.Binding
+	Stumble    key.Binding
 }
 
 var keys = keyMap{
@@ -66,4 +67,5 @@ var keys = keyMap{
 	FocusRight: key.NewBinding(key.WithKeys("l"), key.WithHelp("l", "focus detail")),
 	Tab:        key.NewBinding(key.WithKeys("tab"), key.WithHelp("tab", "focus cycle")),
 	ToggleRail: key.NewBinding(key.WithKeys("ctrl+b"), key.WithHelp("ctrl+b", "toggle tag rail")),
+	Stumble:    key.NewBinding(key.WithKeys("S"), key.WithHelp("S", "stumble")),
 }