feat(tui): add link picker and navigation history

Press [ in detail view to open link picker showing all [[links]]
in the current entry. Enter follows a link, resolving by title
then body substring. Navigation history stack enables esc to pop
back through followed links before returning to list.

Adds Store.ResolveLink() for non-transactional link resolution
from the TUI layer.

internal/tui/commands.go

@@ -59,6 +59,10 @@ type backlinksLoadedMsg struct {
 	backlinks []db.Backlink
 }
 
+type linkFollowedMsg struct {
+	entity *db.Entity
+}
+
 type tagsLoadedMsg struct {
 	tags []db.TagCount
 }
@@ -208,6 +212,16 @@ func loadBacklinks(store *db.Store, entityID string) tea.Cmd {
 	}
 }
 
+func followLink(store *db.Store, linkText string) tea.Cmd {
+	return func() tea.Msg {
+		entity, err := store.ResolveLink(context.Background(), linkText)
+		if err != nil {
+			return errMsg{err}
+		}
+		return linkFollowedMsg{entity}
+	}
+}
+
 func loadRailTags(store *db.Store) tea.Cmd {
 	return func() tea.Msg {
 		tags, err := store.ListTags(context.Background(), false)