Fix launcher/download bugs, add README and Cargo.lock

- launcher: set PROTONPATH to full install path for pinned Proton versions;
  the raw tag name doesn't resolve when umu-run looks it up.
- proton: stream GE-Proton tarballs straight to disk instead of buffering
  ~600 MB in RAM via .bytes(); add error_for_status() on all HTTP calls so
  rate limits and 404s surface clearly; avoid UTF-8 trap on tar args.
- config: fail loudly when $HOME is unset instead of silently writing a Wine
  prefix under /tmp.
- diagnose: replace stat+id shell-out with MetadataExt::uid().
- tray: grab handle() before spawn() consumes the service (the repo didn't
  compile against ksni 0.2 as shipped).
- launcher/diagnose: escape the dot in "battle.net" pgrep patterns so the
  match doesn't false-positive on our own "battlenet-manager" binary; pipe
  pgrep/pkill stdio to /dev/null so PID lists don't leak into our output.
- proton: handle empty release list in pick_interactively cleanly.
- Add README, .gitignore, and commit Cargo.lock for reproducible builds.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/config.rs

@@ -28,9 +28,11 @@ impl Default for Config {
 }
 
 fn home_dir() -> PathBuf {
+    // No sensible fallback — writing a Wine prefix to /tmp would be data-loss
+    // waiting to happen, so surface the misconfiguration instead.
     std::env::var("HOME")
         .map(PathBuf::from)
-        .unwrap_or_else(|_| PathBuf::from("/tmp"))
+        .expect("$HOME is not set; cannot determine default paths")
 }
 
 impl Config {