fix: stabilize Electron startup from source

[zcxGGmu]

Summary

• make source-mode Electron startup default to built renderer assets unless dev server mode is explicitly enabled
• add a dedicated renderer loader plus regression tests for dev-server fallback and quick-task window behavior
• add a beginner-friendly cross-platform guide for building and launching the client from source

Test Plan

• bun test
• bun run --filter='@rv-insights/electron' typecheck
• bun run electron:build