Add CI pipeline, pre-commit hooks, and offline caching

Configure GitHub Actions for linting, typechecking, and testing.
Add Husky and lint-staged for pre-commit checks. Implement API
response caching in AsyncStorage for offline support. Move core
tests to packages/core and add vitest configuration.

.editorconfig

@@ -0,0 +1,19 @@
+# EditorConfig for TimeToLeave
+# https://editorconfig.org
+
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+indent_style = space
+indent_size = 2
+max_line_length = 100
+
+[*.md]
+trim_trailing_whitespace = false
+
+[*.{yml,yaml}]
+indent_size = 2