feat(tui): collapsible tag rail with ambient tag awareness

Persistent left panel showing tags with counts. Provides ambient
awareness of tag landscape without requiring a modal.

- New tagRailModel in tagrail.go: tag list with cursor, scroll, counts
- Rail visible at >=100 cols width, 18% width (min 16 chars)
- ctrl+b toggles rail visibility
- focusTagRail added to focus cycle: capture → tags → list → detail
- j/k navigates, enter filters/unfilters by tag
- Active filter tag highlighted bold in rail
- Tags refresh after entity create/delete/absorb
- Rail auto-hides on narrow terminals, # modal still works as fallback
- Width allocation accounts for rail in split and non-split layouts

internal/tui/tagrail.go

@@ -0,0 +1,138 @@
+package tui
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/lerko/nib/internal/db"
+)
+
+type tagRailModel struct {
+	tags      []db.TagCount
+	cursor    int
+	offset    int
+	height    int
+	width     int
+	activeTag string
+}
+
+func newTagRailModel() tagRailModel {
+	return tagRailModel{}
+}
+
+func (r *tagRailModel) setTags(tags []db.TagCount) {
+	r.tags = tags
+	if r.cursor >= len(tags) {
+		r.cursor = max(0, len(tags)-1)
+	}
+}
+
+func (r *tagRailModel) setSize(width, height int) {
+	r.width = width
+	r.height = height
+}
+
+func (r tagRailModel) selectedTag() string {
+	if len(r.tags) == 0 || r.cursor >= len(r.tags) {
+		return ""
+	}
+	return r.tags[r.cursor].Tag
+}
+
+func (r tagRailModel) update(key string) tagRailModel {
+	switch key {
+	case "up", "k":
+		if r.cursor > 0 {
+			r.cursor--
+			if r.cursor < r.offset {
+				r.offset = r.cursor
+			}
+		}
+	case "down", "j":
+		if r.cursor < len(r.tags)-1 {
+			r.cursor++
+			visible := r.visibleCount()
+			if r.cursor >= r.offset+visible {
+				r.offset = r.cursor - visible + 1
+			}
+		}
+	}
+	return r
+}
+
+func (r tagRailModel) visibleCount() int {
+	v := r.height - 2
+	if v <= 0 {
+		return 10
+	}
+	return v
+}
+
+func (r tagRailModel) view(focused bool) string {
+	w := r.width
+	if w <= 0 {
+		return ""
+	}
+
+	var b strings.Builder
+
+	headerStyle := railHeaderStyle
+	if focused {
+		headerStyle = headerStyle.Foreground(highlight)
+	}
+	b.WriteString(headerStyle.Render("tags"))
+	b.WriteString("\n")
+	b.WriteString(separatorStyle.Render(strings.Repeat("─", w)))
+	b.WriteString("\n")
+
+	if len(r.tags) == 0 {
+		b.WriteString(hintDescStyle.Render(" no tags"))
+		return b.String()
+	}
+
+	visible := r.visibleCount()
+	end := min(r.offset+visible, len(r.tags))
+
+	countW := 0
+	for _, tc := range r.tags {
+		cw := len(fmt.Sprintf("%d", tc.Count))
+		if cw > countW {
+			countW = cw
+		}
+	}
+
+	nameW := w - countW - 3
+	if nameW < 4 {
+		nameW = 4
+	}
+
+	for i := r.offset; i < end; i++ {
+		tc := r.tags[i]
+		name := "#" + tc.Tag
+		if len(name) > nameW {
+			name = name[:nameW-1] + "…"
+		}
+
+		count := fmt.Sprintf("%*d", countW, tc.Count)
+		gap := w - len(name) - len(count) - 1
+		if gap < 1 {
+			gap = 1
+		}
+
+		var line string
+		if i == r.cursor && focused {
+			line = selectedItemStyle.Render(" " + name + strings.Repeat(" ", gap) + railCountStyle.Render(count))
+		} else if tc.Tag == r.activeTag {
+			line = " " + railActiveTagStyle.Render(name) + strings.Repeat(" ", gap) + railCountStyle.Render(count)
+		} else {
+			line = " " + railTagStyle.Render(name) + strings.Repeat(" ", gap) + railCountStyle.Render(count)
+		}
+
+		b.WriteString(line)
+		if i < end-1 {
+			b.WriteString("\n")
+		}
+	}
+
+	return b.String()
+}