feat: add skill installation API endpoint

Add POST /api/skills/install endpoint to install .skill files from
thread's user-data directory. The endpoint extracts the ZIP archive,
validates SKILL.md frontmatter, and installs to skills/custom/.

Frontend Install buttons now call the API instead of downloading.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/core/skills/api.ts

@@ -24,12 +24,39 @@ export async function enableSkill(skillName: string, enabled: boolean) {
   return response.json();
 }
 
-export async function installSkill(skillName: string) {
-  const response = await fetch(
-    `${getBackendBaseURL()}/api/skills/${skillName}/install`,
-    {
-      method: "POST",
+export interface InstallSkillRequest {
+  thread_id: string;
+  path: string;
+}
+
+export interface InstallSkillResponse {
+  success: boolean;
+  skill_name: string;
+  message: string;
+}
+
+export async function installSkill(
+  request: InstallSkillRequest,
+): Promise<InstallSkillResponse> {
+  const response = await fetch(`${getBackendBaseURL()}/api/skills/install`, {
+    method: "POST",
+    headers: {
+      "Content-Type": "application/json",
     },
-  );
+    body: JSON.stringify(request),
+  });
+
+  if (!response.ok) {
+    // Handle HTTP error responses (4xx, 5xx)
+    const errorData = await response.json().catch(() => ({}));
+    const errorMessage =
+      errorData.detail ?? `HTTP ${response.status}: ${response.statusText}`;
+    return {
+      success: false,
+      skill_name: "",
+      message: errorMessage,
+    };
+  }
+
   return response.json();
 }