During the refactor from the monolith into services/, the forge JAR path
had no auto-resolve fallback — unlike lab_jar and forge_decks_dir which
both had resolver functions. When --forge-jar CLI arg and FORGE_JAR env
var are both unset, CFG.forge_jar stayed "" causing POST /api/lab/start
to fail with "Forge JAR not found: ".
Add _resolve_forge_jar() that scans common sibling directories for
forge-gui-desktop-*-jar-with-dependencies.jar (matching the pattern
from start-lab.bat), and wire it as the fallback in main().
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Claude