Add Simplified Chinese translation

[Astyyym]

Summary

• add Simplified Chinese Qt translations for the plugin UI and Processing provider
• make visible UI and Processing strings use Qt translation contexts
• include both editable .ts source and compiled .qm resource
• document the translation compilation command

Validation

• uv run ruff check .
• uv run ruff format --check .
• git diff --check
• compiled i18n/Magicwand_zh.qm with Qt 6 pyside6-lrelease (16 finished translations)
• loaded the .qm with Qt and verified translated UI and Processing strings

qgis_process cannot load this GUI plugin because it supplies no GUI iface; this also occurs with the plugin's existing classFactory() path, so GUI interaction was not exercised through the command-line runner.