feat: Migrate from Manifest V2 to V3

[yize]

Migrate XSwitch from Manifest V2 to V3 for Chrome 88+ compatibility.

Summary

This PR upgrades XSwitch to use Manifest V3, which is required for Chrome 88+.

Changes

manifest.json

• Update manifest_version: 2 → 3
• Replace browser_action with action
• Replace background.scripts with background.service_worker
• Replace webRequest permissions with declarativeNetRequest
• Add host_permissions for all URLs

src/background.ts

• Rewrite using declarativeNetRequest API instead of webRequest
• Service worker pattern instead of persistent background script
• Add updateDeclarativeNetRequestRules() function
• Add updateCorsHeaders() function
• Maintain CORS and cache clearing functionality

build/webpack.v3.config.js (new)

• Webpack configuration for V3 build
• Generates service worker instead of persistent script
• Production mode with minification

package.json

• Add build:v3 script
• Add pub:v3 script

Breaking Changes

• webRequest API replaced with declarativeNetRequest
• Background script now runs as service worker (non-persistent)
• Dynamic request modification now declarative (rules-based)

Testing

Build with: npm run build:v3

API Changes

V2	V3
browser_action	action
webRequest	declarativeNetRequest
blocking callback	declarative rules
persistent script	service worker

Migration Guide

See MANIFEST_V3_MIGRATION_GUIDE.md for detailed migration documentation.