feat(tui): include backlinks in link picker

Link picker now shows both outgoing [[links]] and backlinks in
a unified list with section headers. Backlinks follow by entity
ID directly, outgoing links resolve by text. Navigating into a
backlink works the same as following an outgoing link — pushes
to nav stack, esc pops back.

internal/tui/model.go

@@ -860,7 +860,7 @@ func (m model) updateBrowse(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 
 	case "[":
 		if m.state == stateDetail && m.detail.entity != nil && m.detail.mode == detailPreview {
-			m.linkPicker = newLinkPicker(m.detail.entity.Body)
+			m.linkPicker = newLinkPicker(m.detail.entity.Body, m.detail.backlinks)
 			m.state = stateLinkPicker
 			return m, nil
 		}
@@ -964,14 +964,17 @@ func (m model) updateLinkPicker(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 		m.state = stateDetail
 		return m, nil
 	case "enter":
-		lt := m.linkPicker.selected()
-		if lt == "" {
+		item := m.linkPicker.selected()
+		if item.text == "" && item.entityID == "" {
 			return m, nil
 		}
 		if m.detail.entity != nil {
 			m.navStack = append(m.navStack, m.detail.entity.ID)
 		}
-		return m, followLink(m.store, lt)
+		if item.kind == linkBacklink {
+			return m, followLinkByID(m.store, item.entityID)
+		}
+		return m, followLink(m.store, item.text)
 	default:
 		m.linkPicker = m.linkPicker.update(msg.String())
 		return m, nil