Redesign dashboard top-4 stats as separate cards

Frontend half of the dashboard stat-card redesign, matching the
provided mockup. The 4 metrics used to be plain rows sharing one
bordered box (_add_metric -> a single metric_grid inside one _card).
Replaced with 4 independently-_card-wrapped panels in a responsive
GridContainer (4 cols / 2 compact, same threshold _update_responsive
already used).

Each card now has: a colored icon badge, title + info glyph, a big
value, a colored trend/status line, a small visual, and a footer
caption:
- Question Bank Rating / CCS Rating: a new minimal DashboardSparkline
  (dashboard_sparkline.gd) fed from the current rating and the
  week-ago snapshot (reconstructed client-side from the delta the API
  already returns - no separate history endpoint needed). Trend line
  and caption come from the new delta/percentile fields; both degrade
  to a plain single-point sparkline and a rated-count caption when no
  snapshot exists yet.
- Exam Readiness: a new DashboardSegmentBar (dashboard_segment_bar.gd -
  nothing like a segmented pill progress indicator existed anywhere in
  the codebase) instead of a sparkline, colored by the High/Medium/Low
  label; shows a neutral "complete a simulation" state when the user
  has none yet rather than a fabricated readiness.
- Average Score: reuses the existing ProgressBar/_style_progress,
  relabeled from "First-Attempt Accuracy".

Dropped the old "Study Time" card (not part of the mockup - the
underlying data still drives the weekly activity chart below, it just
no longer gets a dedicated top-line number) and the now-fully-unused
_format_duration.

Verified headlessly: instantiated the real screen against fake
DashboardService data, confirmed every card's value/trend/caption text
and the segment-bar/progress-bar values match the mockup exactly, and
rendered actual screenshots of both the populated and "no data yet"
fallback states.

scripts/ui/dashboard_segment_bar.gd

@@ -0,0 +1,34 @@
+extends Control
+
+const Brand := preload("res://scripts/ui/medmind_style.gd")
+const GAP := 6.0
+const CORNER_RADIUS := 4
+
+var filled_segments := 0
+var total_segments := 4
+var accent := Color.WHITE
+var dark := true
+
+
+func set_progress(next_filled: int, next_total: int, next_accent: Color, dark_mode: bool) -> void:
+	filled_segments = next_filled
+	total_segments = maxi(1, next_total)
+	accent = next_accent
+	dark = dark_mode
+	queue_redraw()
+
+
+func _draw() -> void:
+	var segment_width: float = (size.x - GAP * float(total_segments - 1)) / float(total_segments)
+	if segment_width <= 0.0:
+		return
+	var filled_box := StyleBoxFlat.new()
+	filled_box.bg_color = accent
+	filled_box.set_corner_radius_all(CORNER_RADIUS)
+	var track_box := StyleBoxFlat.new()
+	track_box.bg_color = Brand.muted_color(dark).lerp(Brand.background_color(dark), 0.5)
+	track_box.set_corner_radius_all(CORNER_RADIUS)
+	for index in range(total_segments):
+		var x: float = index * (segment_width + GAP)
+		var rect := Rect2(x, 0.0, segment_width, size.y)
+		draw_style_box(filled_box if index < filled_segments else track_box, rect)