feat: add AI deadline extraction frontend page (Phase 2M)

- ExtractionForm: PDF dropzone (react-dropzone) + text textarea + case selector
- ExtractionResults: review table with edit/remove per row, confidence color-coding
- Page at /ai/extract: upload -> analyze -> review -> adopt deadlines to case
- Extended API client with postFormData for multipart uploads
- Added ExtractedDeadline and ExtractionResponse types

frontend/src/lib/api.ts

@@ -72,6 +72,41 @@ class ApiClient {
   delete<T>(path: string) {
     return this.request<T>(path, { method: "DELETE" });
   }
+
+  async postFormData<T>(path: string, formData: FormData): Promise<T> {
+    const supabase = createClient();
+    const {
+      data: { session },
+    } = await supabase.auth.getSession();
+
+    const headers: HeadersInit = {};
+    if (session?.access_token) {
+      headers["Authorization"] = `Bearer ${session.access_token}`;
+    }
+    const tenantId = typeof window !== "undefined"
+      ? localStorage.getItem("kanzlai_tenant_id")
+      : null;
+    if (tenantId) {
+      headers["X-Tenant-ID"] = tenantId;
+    }
+
+    const res = await fetch(`${this.baseUrl}${path}`, {
+      method: "POST",
+      headers,
+      body: formData,
+    });
+
+    if (!res.ok) {
+      const body = await res.json().catch(() => ({}));
+      const err: ApiError = {
+        error: body.error || res.statusText,
+        status: res.status,
+      };
+      throw err;
+    }
+
+    return res.json();
+  }
 }
 
 export const api = new ApiClient();