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/linkpicker.go

@@ -0,0 +1,65 @@
+package tui
+
+import (
+	"strings"
+
+	"github.com/lerko/nib/internal/link"
+)
+
+type linkPickerModel struct {
+	links  []string
+	cursor int
+}
+
+func newLinkPicker(body string) linkPickerModel {
+	return linkPickerModel{
+		links: link.ExtractLinks(body),
+	}
+}
+
+func (lp linkPickerModel) selected() string {
+	if len(lp.links) == 0 || lp.cursor >= len(lp.links) {
+		return ""
+	}
+	return lp.links[lp.cursor]
+}
+
+func (lp linkPickerModel) update(key string) linkPickerModel {
+	switch key {
+	case "up", "k":
+		if lp.cursor > 0 {
+			lp.cursor--
+		}
+	case "down", "j":
+		if lp.cursor < len(lp.links)-1 {
+			lp.cursor++
+		}
+	}
+	return lp
+}
+
+func (lp linkPickerModel) view(width int) string {
+	var b strings.Builder
+	b.WriteString(titleStyle.Render("follow link") + "\n\n")
+
+	if len(lp.links) == 0 {
+		b.WriteString(hintDescStyle.Render("  no [[links]] in this entry"))
+		b.WriteString("\n\n")
+		b.WriteString(helpStyle.Render("esc:back"))
+		return b.String()
+	}
+
+	for i, lt := range lp.links {
+		label := "[[" + lt + "]]"
+		if i == lp.cursor {
+			b.WriteString(selectedItemStyle.Render(" " + label))
+		} else {
+			b.WriteString(listItemStyle.Render(label))
+		}
+		b.WriteString("\n")
+	}
+
+	b.WriteString("\n")
+	b.WriteString(helpStyle.Render("enter:follow  esc:back"))
+	return b.String()
+}