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/src/app.css

@@ -10,7 +10,8 @@
 	--radius: 0.625rem;
 
 	/* Typography — system stacks, so nothing depends on a CDN or a webfont build. */
-	--app-font-sans: system-ui, -apple-system, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
+	--app-font-sans:
+		system-ui, -apple-system, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
 	--app-font-serif: Georgia, 'Iowan Old Style', 'Times New Roman', serif;
 	--app-font-mono: ui-monospace, 'SF Mono', 'Cascadia Mono', Menlo, Consolas, monospace;