feat: Phase 3 campaign management

Campaign CRUD API with cascading delete across all campaign-scoped
tables, a systems endpoint to seed the create-campaign form, active
campaign state wired to the sidebar chaos factor, and a campaign
management view for creating/switching/deleting campaigns.

public/js/api.js

@@ -1 +1,51 @@
 // Mythic Oracle — shared fetch wrapper, all API calls
+
+const BASE_URL = '/api';
+
+async function request(path, options = {}) {
+  const response = await fetch(`${BASE_URL}${path}`, {
+    headers: { 'Content-Type': 'application/json' },
+    ...options,
+  });
+
+  if (!response.ok) {
+    let message = `Request failed: ${response.status}`;
+    try {
+      const body = await response.json();
+      if (body.error) message = body.error;
+    } catch {
+      // no JSON body on the error response
+    }
+    throw new Error(message);
+  }
+
+  if (response.status === 204) {
+    return null;
+  }
+
+  return response.json();
+}
+
+export function getCampaigns() {
+  return request('/campaigns');
+}
+
+export function createCampaign(data) {
+  return request('/campaigns', { method: 'POST', body: JSON.stringify(data) });
+}
+
+export function getCampaign(id) {
+  return request(`/campaigns/${id}`);
+}
+
+export function updateCampaign(id, data) {
+  return request(`/campaigns/${id}`, { method: 'PATCH', body: JSON.stringify(data) });
+}
+
+export function deleteCampaign(id) {
+  return request(`/campaigns/${id}`, { method: 'DELETE' });
+}
+
+export function getSystems() {
+  return request('/systems');
+}