feat(app): editor configs

[cremetv]

Resolves #499

Description

• Add .editorconfig, .prettierrc, .eslintrc.json, and .stylelintrc.json for consistent code formatting
• Add .vscode/settings.json and .vscode/extensions.json for VS Code workspace configuration
• Install stylelint-config-standard-scss
• refactor app.js and app.scss

These configuration files ensure consistent formatting across all contributors.

File	Purpose
.editorconfig	Editor-agnostic settings (4-space indent etc)
.prettierrc	Code formatter rules
.eslintrc.json	JavaScript linting with browser/Vue globals
.stylelintrc.json	SCSS linting with standard config
.vscode/*	Recommended extensions and workspace settings

Note: the changes in app.js and app.scss are only formatting.