feat: add interactive file search/filter (feature #1)

Pressing / opens a filter bar at the bottom of the screen. As the user
types, the active pane's entries are filtered in real-time using
case-insensitive substring matching. The .. entry is always preserved.

Key bindings:
  /       - open filter input (re-opens with previous query if active)
  Esc     - clear filter and restore full list
  Enter   - commit filter (keeps filtering active, closes input bar)
  Up/Down - navigate the filtered list while typing
  PgUp/Dn - scroll the filtered list

Filter is automatically cleared when navigating to a different directory
(Enter on a folder, Backspace to parent, opening an archive).

Modified files:
  - internal/ui/keymap.go: added Filter key binding (/)
  - internal/ui/model.go: filterMode, filterQuery, filterInput fields;
    filteredPane() helper for View rendering; filter handling in Update();
    renderFilterBar() for the bottom bar UI; clearFilter() helper;
    adjustCursorForFilter() to keep cursor in bounds

Created files:
  - plans/feature-roadmap.md

internal/ui/keymap.go

@@ -24,6 +24,7 @@ type KeyMap struct {
 	Open           key.Binding
 	Back           key.Binding
 	Switch         key.Binding
+	Filter         key.Binding
 	Refresh        key.Binding
 	DirSize        key.Binding
 	Copy           key.Binding
@@ -57,6 +58,7 @@ func DefaultKeyMap() KeyMap {
 		SelectDown:     key.NewBinding(key.WithKeys("shift+down", "J"), key.WithHelp("S-↓/J", "select down")),
 		PageUp:         key.NewBinding(key.WithKeys("pgup"), key.WithHelp("PgUp", "page up")),
 		PageDown:       key.NewBinding(),
+		Filter:         key.NewBinding(key.WithKeys("/"), key.WithHelp("/", "filter")),
 		Open:           key.NewBinding(key.WithKeys("enter", "right"), key.WithHelp("Enter", "open")),
 		Back:           key.NewBinding(key.WithKeys("backspace", "left"), key.WithHelp("←", "parent")),
 		Switch:         key.NewBinding(key.WithKeys("tab", "h", "l"), key.WithHelp("Tab/h/l", "switch pane")),