feat(tui): add status bar, help overlay, tag filter, and entity actions

Status bar with entity count and context-sensitive key hints. Help
overlay via ? key. Tag filter via # with cursor-navigable tag list.
Todo toggle (x), pin (!), promote (p), demote (D), copy (c), edit (e)
via $EDITOR. Delete confirmation with 3s timeout. Date-grouped list
with completed todo and pinned indicators. Esc clears active tag filter.

Adds CompletedAt/ClearCompleted to EntityUpdate for todo toggling.

internal/tui/filter.go

@@ -0,0 +1,84 @@
+package tui
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/lerko/nib/internal/db"
+)
+
+type filterModel struct {
+	tags   []db.TagCount
+	cursor int
+	height int
+}
+
+func newFilterModel() filterModel {
+	return filterModel{}
+}
+
+func (f *filterModel) setTags(tags []db.TagCount) {
+	f.tags = tags
+	f.cursor = 0
+}
+
+func (f *filterModel) setHeight(h int) {
+	f.height = h
+}
+
+func (f filterModel) selectedTag() string {
+	if len(f.tags) == 0 || f.cursor >= len(f.tags) {
+		return ""
+	}
+	return f.tags[f.cursor].Tag
+}
+
+func (f filterModel) update(key string) filterModel {
+	switch key {
+	case "up", "k":
+		if f.cursor > 0 {
+			f.cursor--
+		}
+	case "down", "j":
+		if f.cursor < len(f.tags)-1 {
+			f.cursor++
+		}
+	}
+	return f
+}
+
+func (f filterModel) view(width int) string {
+	if len(f.tags) == 0 {
+		return statusStyle.Render("no tags")
+	}
+
+	var b strings.Builder
+	b.WriteString(titleStyle.Render("filter by tag"))
+	b.WriteString("\n\n")
+
+	visible := f.height - 4
+	if visible <= 0 {
+		visible = 10
+	}
+
+	offset := 0
+	if f.cursor >= visible {
+		offset = f.cursor - visible + 1
+	}
+	end := min(offset+visible, len(f.tags))
+
+	for i := offset; i < end; i++ {
+		tc := f.tags[i]
+		tag := fmt.Sprintf("#%-20s %d", tc.Tag, tc.Count)
+		if i == f.cursor {
+			b.WriteString(selectedItemStyle.Render(" " + tagStyle.Render(tag)))
+		} else {
+			b.WriteString(listItemStyle.Render(tagStyle.Render(tag)))
+		}
+		if i < end-1 {
+			b.WriteString("\n")
+		}
+	}
+
+	return b.String()
+}