chore: add ESLint, Prettier, and Vitest tooling

Add code-quality tooling the project lacked:
- ESLint (flat config via @sveltejs/eslint-config)
- Prettier (tabs, single quotes, matching existing style)
- Vitest (node env, for pure-logic unit tests)
- npm scripts: lint, format, format:check, test, test:watch

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.prettierrc

@@ -0,0 +1,15 @@
+{
+	"useTabs": true,
+	"singleQuote": true,
+	"trailingComma": "none",
+	"printWidth": 100,
+	"plugins": ["prettier-plugin-svelte"],
+	"overrides": [
+		{
+			"files": "*.svelte",
+			"options": {
+				"parser": "svelte"
+			}
+		}
+	]
+}