feat(omnitools): 添加 OmniTools 应用配置及文档

apps/omnitools/README_en.md

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+## OmniTools
+
+**OmniTools** is a fully client-side toolkit for everyday tasks across media, PDF, text, time, math, and data. Files are processed **in-browser**, with **no ads** and **no tracking**. The Docker image is about **28MB** for fast deployment.
+
+### Features
+- **Image/Video/Audio**: resize, convert, edit, trim, reverse
+- **PDF**: split, merge, edit
+- **Text/List**: case converters, list shuffler, formatters
+- **Date & Time**: calculators, time zone converters
+- **Math**: prime generation, voltage/current/resistance calculations
+- **Data**: tools for JSON, CSV, XML
+
+### Project Snapshot
+- Demo: [omnitools.app](https://omnitools.app)