chore: format the tree and clear ruff's findings

Applies ruff format, ruff check --fix and prettier, so CI can gate on all three.
The surviving unused imports were all re-exports in __init__.py, now covered by a
per-file ignore; the pdf.js viewer and the generated openapi types join the
vendored code that eslint and prettier already skip.

frontend/eslint.config.js

@@ -13,7 +13,9 @@ const gitignorePath = fileURLToPath(new URL('./.gitignore', import.meta.url));
 export default defineConfig(
 	includeIgnoreFile(gitignorePath),
 	// Vendored third-party source. Tracked, so .gitignore does not cover it.
-	{ ignores: ['src/lib/vendor/**'] },
+	// static/pdfjs is the pdf.js viewer, vendored the same way as src/lib/vendor/foliate-js;
+	// linting it produced 1717 of the 1804 errors this config used to report.
+	{ ignores: ['src/lib/vendor/**', 'static/pdfjs/**'] },
 	js.configs.recommended,
 	...ts.configs.recommended,
 	...svelte.configs.recommended,