Add floating canvas panels, theme settings, and SVG line-spacing analysis.

Canvas Studio now uses dockable floating panels, app settings/help navigation, and improved SVG export; the backend adds an SVG line-spacing analysis API with SciPy acceleration and new design templates.

frontend/src/lib/api.ts

@@ -0,0 +1,33 @@
+const rawApiBase = (import.meta.env.VITE_API_BASE ?? '').trim();
+
+export const API_BASE = rawApiBase.replace(/\/+$/, '');
+
+export function apiUrl(path: string) {
+  if (!path) return API_BASE || '';
+  if (/^https?:\/\//i.test(path) || path.startsWith('data:') || path.startsWith('blob:')) {
+    return path;
+  }
+  const normalizedPath = path.startsWith('/') ? path : `/${path}`;
+  return `${API_BASE}${normalizedPath}`;
+}
+
+export function apiEventSource(path: string) {
+  return new EventSource(apiUrl(path));
+}
+
+export async function readApiError(res: Response) {
+  const text = await res.text().catch(() => '');
+  if (!text) return `${res.status} ${res.statusText}`.trim();
+  try {
+    const parsed = JSON.parse(text);
+    return parsed.detail || parsed.message || text;
+  } catch {
+    return text;
+  }
+}
+
+export async function ensureOk(res: Response, fallback: string) {
+  if (res.ok) return res;
+  const detail = await readApiError(res);
+  throw new Error(`${fallback} (${res.status})${detail ? `: ${detail}` : ''}`);
+}