Follow System Locale with Optional Override

### Describe your improvement idea or feature request

Allow the Taskwarrior Flutter app to automatically follow the device system language by default, with an optional toggle in Settings to manually override the language.
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## Problem

Currently, the app does not automatically follow the device's system locale.

Most modern mobile applications:
- Use the system language by default
- Allow users to manually override it in settings

Without this behavior:
- The app may not feel native
- International users may experience friction
- Localization support is limited in usability

### How would you like to see it implemented?

## Proposed Solution

### 1. Follow System Locale by Default

If no language is manually selected:
- The app should use the device `Locale`
- If the locale is unsupported, fallback to English

Example:
- Device language: `pa` (Punjabi) → App uses Punjabi (if supported)
- Unsupported language → Fallback to English

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### 2. Add “Follow System Language” Toggle

In Settings:

### Put here any screenshots or videos (optional)

_No response_

### How can we contact you (optional)

_No response_

### Would you like to work on this issue?

Yes

### By submitting this issue, I have confirmed that:

- [x] I have starred the repo ⭐ and watched 👀 it on GitHub and followed the contribution guidelines.

Start by tracing the app's current localization entry point and the Settings screen, then inspect how supported locales and the device Locale are handled. Done means the app follows the system language by default, falls back to English when unsupported, and lets users override that choice from Settings.

Written by the indexing model from the issue text.