Add Chinese/English language switch in settings

[Dai666-me]

What does this PR do?

Adds a language switcher in the Settings page, supporting Chinese (Simplified) and English.
All UI texts (including status, buttons, settings labels, help modal, footer, etc.) can be toggled between the two languages without reloading the page. The selected language is saved in localStorage and persists after refresh.

Key changes

• New feature: Language toggle with two buttons (中文 / English) placed in Settings page.
• i18n data: Embedded inside app.js (no extra HTTP request) for faster loading.
• Dynamic translation: Uses data-i18n attributes in HTML; JS function applyLanguage() updates all texts on switch.
• Preserved original behavior: All existing functionality (start/stop, settings save, stats, logs, etc.) unchanged.

Files modified

• index.html: Added language switch UI, added data-i18n to all translatable elements.
• app.js: Added langData object and applyLanguage(); modified checkStatus(), controlAdGuard(), saveSettings(), loadLog(), collectDebugInfo() to use translated strings; added language switching event binding.

Testing performed

• Tested on real device with KernelSU WebUI.
• Switching between Chinese and English updates every UI element correctly.
• After refresh, language choice is remembered.
• No regression on module control (start/stop/restart) or statistics fetching.

Additional notes

• This PR does not modify style.css or any existing logic structure.
• Translation keys are minimal (only what appears on screen). Future language additions can be done by extending langData.

Screenshots