chore: add basic copilot instructions

.github/copilot-instructions.md

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+# Core Requirements
+
+- The end goal is stability, speed and great user experience.
+
+## Code Quality Requirements
+
+- Follow standard TypeScript conventions and best practices
+- Use the Composition API when creating Vue components
+- Use clear, descriptive variable and function names
+- Add comments to explain complex logic or non-obvious implementations
+- Write unit tests for core functionality using `vitest`
+- Write end-to-end tests using Playwright and `@nuxt/test-utils`
+- Keep functions focused and manageable (generally under 50 lines)
+- Use error handling patterns consistently