refactor: simplify TUI theme struct and extract chart/progress components

Theme simplification (theme/theme.go):
- Remove Purple and BorderHover fields from Theme struct — neither was
  referenced in the new tab renderers, reducing per-theme boilerplate
  from 14 to 12 color definitions

Component extraction (components/):
- Move Sparkline() and BarChart() from card.go to new chart.go, giving
  visualization components their own file as complexity grows
- card.go retains MetricCard, ContentCard, LayoutRow, and CardInnerWidth
  which are layout-focused
- New chart.go: Sparkline (unicode block chars) and BarChart (multi-row
  with anchored Y-axis, optional X-axis labels, dynamic height/width)
- New progress.go: ProgressBar component with customizable width, color,
  and percentage display — used by rate-limit and budget views

Status bar and tab bar updates:
- statusbar.go: adapt to simplified theme struct
- tabbar.go: adapt to simplified theme struct

internal/tui/components/progress.go

@@ -0,0 +1,142 @@
+package components
+
+import (
+	"fmt"
+	"strings"
+	"time"
+
+	"cburn/internal/tui/theme"
+
+	"github.com/charmbracelet/bubbles/progress"
+	"github.com/charmbracelet/lipgloss"
+)
+
+// 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)
+
+	var b strings.Builder
+	for i := 0; i < filled; i++ {
+		b.WriteString(filledStyle.Render("\u2588"))
+	}
+	for i := filled; i < width; i++ {
+		b.WriteString(emptyStyle.Render("\u2591"))
+	}
+
+	return fmt.Sprintf("%s %.1f%%", b.String(), pct*100)
+}
+
+// ColorForPct returns green/yellow/red based on utilization level.
+func ColorForPct(pct float64) string {
+	t := theme.Active
+	switch {
+	case pct >= 0.8:
+		return string(t.Red)
+	case pct >= 0.5:
+		return string(t.Orange)
+	default:
+		return string(t.Green)
+	}
+}
+
+// RateLimitBar renders a labeled progress bar with percentage and countdown.
+// label: "5-hour", "Weekly", etc.  pct: 0.0-1.0.  resetsAt: zero means no countdown.
+// barWidth: width allocated for the progress bar portion only.
+func RateLimitBar(label string, pct float64, resetsAt time.Time, labelW, barWidth int) string {
+	t := theme.Active
+
+	if pct < 0 {
+		pct = 0
+	}
+	if pct > 1 {
+		pct = 1
+	}
+
+	bar := progress.New(
+		progress.WithSolidFill(ColorForPct(pct)),
+		progress.WithWidth(barWidth),
+		progress.WithoutPercentage(),
+	)
+	bar.EmptyColor = string(t.TextDim)
+
+	labelStyle := lipgloss.NewStyle().Foreground(t.TextMuted)
+	pctStyle := lipgloss.NewStyle().Foreground(t.TextPrimary).Bold(true)
+	countdownStyle := lipgloss.NewStyle().Foreground(t.TextDim)
+
+	pctStr := fmt.Sprintf("%3.0f%%", pct*100)
+	countdown := ""
+	if !resetsAt.IsZero() {
+		dur := time.Until(resetsAt)
+		if dur > 0 {
+			countdown = formatCountdown(dur)
+		} else {
+			countdown = "now"
+		}
+	}
+
+	return fmt.Sprintf("%s %s %s  %s",
+		labelStyle.Render(fmt.Sprintf("%-*s", labelW, label)),
+		bar.ViewAs(pct),
+		pctStyle.Render(pctStr),
+		countdownStyle.Render(countdown),
+	)
+}
+
+// CompactRateBar renders a tiny status-bar-sized rate indicator.
+// Example output: "5h ████░░░░ 42%"
+func CompactRateBar(label string, pct float64, width int) string {
+	t := theme.Active
+
+	if pct < 0 {
+		pct = 0
+	}
+	if pct > 1 {
+		pct = 1
+	}
+
+	// label + space + bar + space + pct(4 chars)
+	barW := width - lipgloss.Width(label) - 6
+	if barW < 4 {
+		barW = 4
+	}
+
+	bar := progress.New(
+		progress.WithSolidFill(ColorForPct(pct)),
+		progress.WithWidth(barW),
+		progress.WithoutPercentage(),
+	)
+	bar.EmptyColor = string(t.TextDim)
+
+	pctStyle := lipgloss.NewStyle().Foreground(lipgloss.Color(ColorForPct(pct)))
+	labelStyle := lipgloss.NewStyle().Foreground(t.TextMuted)
+
+	return fmt.Sprintf("%s %s %s",
+		labelStyle.Render(label),
+		bar.ViewAs(pct),
+		pctStyle.Render(fmt.Sprintf("%2.0f%%", pct*100)),
+	)
+}
+
+func formatCountdown(d time.Duration) string {
+	h := int(d.Hours())
+	m := int(d.Minutes()) % 60
+	if h >= 24 {
+		days := h / 24
+		hours := h % 24
+		return fmt.Sprintf("%dd %dh", days, hours)
+	}
+	if h > 0 {
+		return fmt.Sprintf("%dh %dm", h, m)
+	}
+	return fmt.Sprintf("%dm", m)
+}