feat: add TUI reusable components: metric cards, sparklines, tab bar, and status bar

Implement shared UI components used across the dashboard tabs:

- tui/components/card.go: Three components:

  * MetricCard: bordered card with label (muted), value (bold), and
    optional delta (dim) — used for the dashboard's top-level KPIs.

  * MetricCardRow: renders N cards side-by-side using lipgloss
    horizontal join, auto-calculating card width from available space.

  * Sparkline: theme-colored Unicode block sparkline (8-level,
    auto-scaled to series max). Used in Dashboard and Trends tabs.

  * ProgressBar: filled/empty bar (accent + dim) with percentage
    label. Used in the Budget tab for plan-relative spend.

- tui/components/statusbar.go: Bottom status bar with left-aligned
  keybinding hints ([f]ilter [?]help [q]uit), current filter info,
  and right-aligned data age indicator. Padding auto-fills to
  terminal width.

- tui/components/tabbar.go: 10-tab navigation bar split across two
  rows (Dashboard/Costs/Sessions/Models/Projects on row 1,
  Trends/Efficiency/Activity/Budget/Settings on row 2). Each
  inactive tab highlights its keyboard shortcut letter with [bracket]
  notation. Active tab renders in accent color. Settings uses 'x'
  as its shortcut (not present in the name, so appended). TabIdxByKey
  maps key presses to tab indices.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/tui/components/card.go

@@ -0,0 +1,117 @@
+package components
+
+import (
+	"fmt"
+
+	"cburn/internal/tui/theme"
+
+	"github.com/charmbracelet/lipgloss"
+)
+
+// MetricCard renders a small metric card with label, value, and delta.
+func MetricCard(label, value, delta string, width int) string {
+	t := theme.Active
+
+	cardStyle := lipgloss.NewStyle().
+		Border(lipgloss.RoundedBorder()).
+		BorderForeground(t.Border).
+		Width(width).
+		Padding(0, 1)
+
+	labelStyle := lipgloss.NewStyle().
+		Foreground(t.TextMuted)
+
+	valueStyle := lipgloss.NewStyle().
+		Foreground(t.TextPrimary).
+		Bold(true)
+
+	deltaStyle := lipgloss.NewStyle().
+		Foreground(t.TextDim)
+
+	content := labelStyle.Render(label) + "\n" +
+		valueStyle.Render(value)
+	if delta != "" {
+		content += "\n" + deltaStyle.Render(delta)
+	}
+
+	return cardStyle.Render(content)
+}
+
+// MetricCardRow renders a row of metric cards side by side.
+func MetricCardRow(cards []struct{ Label, Value, Delta string }, totalWidth int) string {
+	if len(cards) == 0 {
+		return ""
+	}
+
+	cardWidth := (totalWidth - len(cards) - 1) / len(cards)
+	if cardWidth < 10 {
+		cardWidth = 10
+	}
+
+	var rendered []string
+	for _, c := range cards {
+		rendered = append(rendered, MetricCard(c.Label, c.Value, c.Delta, cardWidth))
+	}
+
+	return lipgloss.JoinHorizontal(lipgloss.Top, rendered...)
+}
+
+// Sparkline renders a unicode sparkline from values.
+func Sparkline(values []float64, color lipgloss.Color) string {
+	if len(values) == 0 {
+		return ""
+	}
+
+	blocks := []rune{'▁', '▂', '▃', '▄', '▅', '▆', '▇', '█'}
+
+	max := values[0]
+	for _, v := range values[1:] {
+		if v > max {
+			max = v
+		}
+	}
+	if max == 0 {
+		max = 1
+	}
+
+	style := lipgloss.NewStyle().Foreground(color)
+
+	var result string
+	for _, v := range values {
+		idx := int(v / max * float64(len(blocks)-1))
+		if idx >= len(blocks) {
+			idx = len(blocks) - 1
+		}
+		if idx < 0 {
+			idx = 0
+		}
+		result += string(blocks[idx])
+	}
+
+	return style.Render(result)
+}
+
+// ProgressBar renders a colored progress bar.
+func ProgressBar(pct float64, width int) string {
+	t := theme.Active
+	filled := int(pct * float64(width))
+	if filled > width {
+		filled = width
+	}
+	if filled < 0 {
+		filled = 0
+	}
+
+	filledStyle := lipgloss.NewStyle().Foreground(t.Accent)
+	emptyStyle := lipgloss.NewStyle().Foreground(t.TextDim)
+
+	bar := ""
+	for i := 0; i < filled; i++ {
+		bar += filledStyle.Render("█")
+	}
+	for i := filled; i < width; i++ {
+		bar += emptyStyle.Render("░")
+	}
+
+	return fmt.Sprintf("%s %.1f%%", bar, pct*100)
+}