Polish for shipping: robust auto-detect, empty state, real icons, end-user README

- cli_usage::default_command now enumerates WSL distros and probes each for
  claude before falling back; no more hardcoded -d Ubuntu.
- New autodetect_claude_command Tauri command + IPC binding so the UI knows
  whether claude is reachable.
- App.svelte: clear 'Claude Code not found' empty state with install link.
- Real icons: scripts/make-icon.py generates a 1024x1024 source.png; runtime
  produces 32/128/256 PNGs and a multi-resolution .ico. README in icons/
  explains how to regen.
- README rewritten for friends: install / requirements / troubleshooting on
  top; build-from-source moved to bottom.

scripts/make-icon.py

@@ -0,0 +1,72 @@
+"""
+Regenerate src-tauri/icons/source.png — dark rounded square + purple
+progress ring + white 'C'. Matches the running widget's visual language.
+
+Run from the project root:
+    python3 scripts/make-icon.py
+    pnpm tauri icon src-tauri/icons/source.png   # generates ico/icns/etc
+"""
+
+import math
+import os
+import sys
+from pathlib import Path
+
+try:
+    from PIL import Image, ImageDraw, ImageFont
+except ImportError:
+    sys.stderr.write("Pillow not installed: pip install --user Pillow\n")
+    sys.exit(2)
+
+SIZE = 1024
+PAD = 40
+RADIUS = 180
+RING_PAD = 200
+STROKE = 60
+PROGRESS = 0.72  # decorative; matches the widget at ~real usage
+
+BG = (22, 24, 32, 255)
+TRACK = (255, 255, 255, 26)
+ACCENT = (176, 139, 255, 255)
+FG = (232, 234, 240, 255)
+
+
+def main(out: Path) -> None:
+    img = Image.new("RGBA", (SIZE, SIZE), (0, 0, 0, 0))
+    draw = ImageDraw.Draw(img)
+
+    draw.rounded_rectangle([PAD, PAD, SIZE - PAD, SIZE - PAD], radius=RADIUS, fill=BG)
+
+    box = [RING_PAD, RING_PAD, SIZE - RING_PAD, SIZE - RING_PAD]
+    draw.arc(box, start=0, end=360, fill=TRACK, width=STROKE)
+
+    sweep = 360 * PROGRESS
+    draw.arc(box, start=-90, end=-90 + sweep, fill=ACCENT, width=STROKE)
+
+    end_rad = math.radians(-90 + sweep)
+    cx, cy = SIZE // 2, SIZE // 2
+    r = (SIZE - 2 * RING_PAD) / 2
+    ex, ey = cx + r * math.cos(end_rad), cy + r * math.sin(end_rad)
+    dot = STROKE // 2
+    draw.ellipse([ex - dot, ey - dot, ex + dot, ey + dot], fill=ACCENT)
+
+    try:
+        font = ImageFont.load_default(size=320)
+    except Exception:
+        font = None
+    if font:
+        text = "C"
+        bbox = draw.textbbox((0, 0), text, font=font)
+        tw = bbox[2] - bbox[0]
+        th = bbox[3] - bbox[1]
+        draw.text(((SIZE - tw) / 2 - bbox[0], (SIZE - th) / 2 - bbox[1] - 12),
+                  text, font=font, fill=FG)
+
+    out.parent.mkdir(parents=True, exist_ok=True)
+    img.save(out, "PNG")
+    print(f"wrote {out} {SIZE}x{SIZE}")
+
+
+if __name__ == "__main__":
+    here = Path(__file__).resolve().parent.parent
+    main(here / "src-tauri" / "icons" / "source.png")