chore: adjust entrypoint files and their styles

[eglitise]

This PR has a few changes for the HTML entrypoint files and their imported style files:

• Move entrypoint files back to common directory (partial revert of chore: separate web and Electron-specific code #1514)
  • After more investigation into Vite, it made more sense to use the common directory as the root folder, and having the entrypoint file be outside the root was causing issues
• Adjust the title to 'Appium Inspector'
• Change the splash window's .less file to .css
  • The splash window does not use any antd-specific CSS, so there is no need to import it. This slightly reduces the bundle size, and allows to skip the less->css translation process during build (for this file only)
• Merge app.global.less and common.global.less into main.less

As usual, both the browser and Electron builds were tested to work fine.