add i18n zh-CN

[sky22333]

Summary

• Add Simplified Chinese (values-zh-rCN/strings.xml) for all existing UI strings.
• Rely on Android’s built-in locale resolution: zh-CN shows Chinese; all other locales fall back to English in values/.
• No runtime locale switching or code changes; layout and dynamic text already use string resources.

Testing

• Verified all 61 string keys match between values/strings.xml and values-zh-rCN/strings.xml.
• Manual: set device language to Simplified Chinese and confirm UI shows Chinese on first launch (no English flash).
• Manual: set device language to English (or an unsupported locale) and confirm UI stays in English.
• Manual: change system language while the app is open and confirm the activity recreates with the correct language.

F-Droid / Source Build Impact

• No new bundled binaries or generated assets
• Any new generated asset can be built from source
• No new network dependency, telemetry, ads, or closed-source component

USB Safety Impact

• No change to destructive USB write behavior
• Destructive write behavior changed and is described above