feat: show window instantly with cached theme background

[oahsiao]

Summary

Speeds up the perceived cold-start of the app by showing the BrowserWindow
the moment it is created — instead of waiting for the renderer to finish its
first paint — and seeds it with the correct theme background so dark-theme
users no longer see a white flash before the UI appears.

The window background color is read from a small cache (startup.json) that the
renderer writes every time a theme is applied, so the next cold start paints with
the right color immediately.

Changes

File	Change
src/main/main.js	New startupBackground() helper reads userData/startup.json → bg, validates against /^#[0-9a-fA-F]{3,8}$/, falls back to default theme bg #f4f4f4. Window now uses explicit show: true + backgroundColor: startupBackground(). Wrapped in try/catch so first launch (no cache) never throws.
src/main/ipc.js	New app:setStartupBg handler: validates hex color (same regex), writes { "bg": <color> } to userData/startup.json via fs.writeFileSync, returns true/false. Full try/catch. Added app to the electron import.
src/preload/preload.js	Exposes setStartupBg: (color) => ipcRenderer.invoke('app:setStartupBg', color) on the existing m2scout contextBridge API. contextIsolation: true / nodeIntegration: false unchanged.
src/renderer/js/themes.js	In apply(), after setting CSS variables, caches the current vars['--bg'] via window.m2scout.setStartupBg(...) (existence-checked + try/catch). Runs on every theme switch and on init().

How it works

1. Renderer applies a theme → caches its --bg to userData/startup.json.
2. Next cold start → main process reads that file and paints the window with the
   correct color immediately, then shows content as the renderer loads.

What this does NOT fix (by design / out of scope)

• Eliminates the wait for the renderer's first full paint → window appears
  earlier; content fills in right after.
• The Electron runtime cold-start and Windows Defender / SmartScreen scanning
  of electron.exe cannot be removed by app code.
• npm start (electron .) is slower than running
  node_modules/electron/dist/electron.exe directly (extra npm/node layer);
  a packaged asar build is typically fastest.
• app.getPath('userData') may point to an app-name folder in dev mode; set and
  get use the same path, so it stays consistent.

Testing

• node --check on all four modified files → OK.
• npm test → 5 pass / 0 fail.
• Smoke test: app launched for ~6s, no stderr, clean exit.