Remove standalone fetcher, add setup tool with install/uninstall workflow

Drop claude-fetcher binary and cron job — the widget's built-in
BackgroundFetcher is the sole fetcher now. Add cmd/setup with cross-platform
install and uninstall (--uninstall): kills widget, removes binaries + autostart,
cleans Claude Code statusline setting, optionally removes config dir.

Also includes: browser-based login (chromedp), ICO wrapper for Windows tray
icon, and reduced icon size (64px).

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

.gitignore

@@ -1,7 +1,7 @@
 # Go binaries
 claude-statusline
-claude-fetcher
 claude-widget
+/setup
 *.exe
 
 # Build output