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.

src-tauri/icons/README.md

@@ -1,13 +1,19 @@
 # Icons
 
-Placeholder. Before `pnpm tauri build` will succeed you need real icons here.
+`source.png` — 1024×1024 master icon. Dark rounded square + purple progress
+ring + white "C". Generated by `../../scripts/make-icon.py`.
 
-Quickest path: `pnpm tauri icon path/to/source-1024x1024.png` — Tauri generates every required size + format (`.ico`, `.icns`, `.png`).
+To regenerate every required size + format Tauri's bundler needs:
 
-Files Tauri's bundler expects (referenced from `tauri.conf.json`):
+```sh
+pnpm tauri icon src-tauri/icons/source.png
+```
 
-- `32x32.png`
-- `128x128.png`
-- `128x128@2x.png`
-- `icon.icns`
-- `icon.ico`
+That populates `32x32.png`, `128x128.png`, `128x128@2x.png`, `icon.icns`,
+`icon.ico`, plus Android/iOS sizes (we ignore those — desktop only).
+
+The generated icons are tracked in git so a clean clone can `pnpm tauri build`
+without first running `tauri icon`.
+
+To customize: edit `scripts/make-icon.py` (colors, progress sweep, monogram)
+and rerun.