feat(theme): add Vazirmatn font and auto-apply it for Persian locale

### What’s new - **Add Vazirmatn font** (`Vazirmatn-Regular.ttf`, `Vazirmatn-Bold.ttf`) under `assets/fonts/`. - Register the font in **pubspec.yaml**. - **Theme update:** `fontFamily` now switches to *Vazirmatn* whenever `locale.languageCode == 'fa'`; other locales keep their default/system font. - Include **SIL Open Font License** file for Vazirmatn. > **Note:** This change only activates the Vazirmatn font for Persian UI. > If there’s any issue with this PR, please don’t reject it outright—feel free to make small adjustments to get it over the line. > The system default font for Persian on some OSes isn’t optimal or beautiful, and Vazirmatn (an open-source Persian font) offers a much better experience. The license file is included. --- ### Testing I’m not yet familiar with the project’s build/test workflow, so I haven’t been able to verify this locally. **Please publish this as a beta update** so I can download, test it in my environment, and address any issues that arise. --- ### Checklist - [x] Font files added to repo - [x] Font declared in `pubspec.yaml` - [x] License file added - [ ] Verified on Persian locale (pending beta test)