fix: handle renderer startup load failures

[itzptk]

Summary

• route all main/tray renderer loads through a shared diagnostic loader that logs window, route, target URL/file, and serialized error before rethrowing
• add top-level bootstrap failure logging so startup rejections no longer disappear as unhandled promises
• keep window creation/lifecycle setup synchronous, register IPC before awaited startup renderer loads, and cover load-failure logging with Vitest

Verification

• git diff --check
• npm run test:node -- renderer-loader
• npm run test:node
• npm run test:all
• npm run build
• independent review: passed after follow-up fix

Manual invalid ELECTRON_START_URL app launch was attempted, but this Linux runner has no X server/display, so Electron exits before app code reaches renderer loading.

Closes #49