ui: chart polish — rotated y-axis labels, wider viewBox, single-day fallback

- Add rotated 'Size' (left) and 'Snapshots' (right) axis titles in
  the chart's outer margins so the two y-axes are self-describing.
- Bump the chart viewBox from 600x220 to 640x220 and lift padL from
  56 to 72 so the rotated labels and byte tick numbers don't crowd.
- Dedupe the X-axis labels for short windows (1 or 2 days collapsed
  the start/mid/end indices onto each other, stacking 'May 7' three
  times); the 1-day case now centres a single label, 2-day uses
  start+end only.
- Pin a lone data dot to the chart centre instead of the left edge
  when len(days)==1, so it sits under the centred date label.

Goldens regenerated.

internal/web/sparkline/sparkline.go

@@ -156,7 +156,7 @@ func RenderChart(series []Series, days []time.Time, opts ChartOpts) template.HTM
 	if opts.EmptyLabel == "" {
 		opts.EmptyLabel = "no data yet"
 	}
-	const padL, padR, padT, padB = 56, 56, 16, 28
+	const padL, padR, padT, padB = 72, 56, 16, 28
 	w, h := opts.Width, opts.Height
 	innerW := w - padL - padR
 	innerH := h - padT - padB
@@ -255,6 +255,11 @@ func RenderChart(series []Series, days []time.Time, opts ChartOpts) template.HTM
 				continue
 			}
 			x := float64(padL) + stepX*float64(i)
+			if len(days) == 1 {
+				// Single-day: pin the lone dot to the chart centre so it
+				// sits under the centred date label.
+				x = float64(padL) + float64(innerW)/2
+			}
 			y := float64(padT) + float64(innerH) - (v-a.min)/(a.max-a.min)*float64(innerH)
 			fmt.Fprintf(&seg, "%.2f,%.2f ", x, y)
 			d := days[i]
@@ -267,18 +272,48 @@ func RenderChart(series []Series, days []time.Time, opts ChartOpts) template.HTM
 
 	if axArr[AxisLeft].has {
 		writeAxisLabels(&b, padL-6, padT, innerH, axArr[AxisLeft].min, axArr[AxisLeft].max, FormatBytes, "end")
+		// Rotated axis title in the left margin. Position inset from
+		// the viewBox edge by ≈ font-size so the rotated glyph extents
+		// don't clip against the SVG boundary.
+		cy := padT + innerH/2
+		fmt.Fprintf(&b,
+			`<text x="14" y="%d" text-anchor="middle" font-size="11" fill="currentColor" fill-opacity="0.55" transform="rotate(-90, 14, %d)">Size</text>`,
+			cy, cy)
 	}
 	if axArr[AxisRight].has {
 		writeAxisLabels(&b, w-padR+6, padT, innerH, axArr[AxisRight].min, axArr[AxisRight].max, FormatCount, "start")
+		cy := padT + innerH/2
+		fmt.Fprintf(&b,
+			`<text x="%d" y="%d" text-anchor="middle" font-size="11" fill="currentColor" fill-opacity="0.55" transform="rotate(90, %d, %d)">Snapshots</text>`,
+			w-14, cy, w-14, cy)
 	}
 
-	xLabels := []int{0, len(days) / 2, len(days) - 1}
-	anchors := []string{"start", "middle", "end"}
-	for i, idx := range xLabels {
-		x := float64(padL) + stepX*float64(idx)
+	// X-axis labels at start / mid / end. With 1-2 days the indices
+	// collapse onto each other — dedupe so we don't stack overlapping
+	// "Jan 2" labels at the same x coordinate.
+	type xLabel struct {
+		idx    int
+		anchor string
+	}
+	var xLabels []xLabel
+	switch {
+	case len(days) == 1:
+		xLabels = []xLabel{{0, "middle"}}
+	case len(days) == 2:
+		xLabels = []xLabel{{0, "start"}, {1, "end"}}
+	default:
+		xLabels = []xLabel{{0, "start"}, {len(days) / 2, "middle"}, {len(days) - 1, "end"}}
+	}
+	for _, l := range xLabels {
+		x := float64(padL) + stepX*float64(l.idx)
+		// With a single point, anchor "middle" centres on padL — push to
+		// the chart's centre line so the lone label sits over the dot.
+		if len(days) == 1 {
+			x = float64(padL) + float64(innerW)/2
+		}
 		fmt.Fprintf(&b,
 			`<text x="%.2f" y="%d" text-anchor="%s" font-size="10" fill="currentColor" fill-opacity="0.55">%s</text>`,
-			x, h-padB+16, anchors[i], days[idx].Format("Jan 2"))
+			x, h-padB+16, l.anchor, days[l.idx].Format("Jan 2"))
 	}
 
 	b.WriteString(`</svg>`)