feat(admin): collapse create-feed form into accordion

Wrap the "Create New Feed" form in a native <details> accordion, collapsed
by default and auto-opened when no feeds exist. After creating a feed,
redirect to the "Your Feeds" anchor so the new feed is immediately visible.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/routes/admin.test.ts

@@ -148,7 +148,7 @@ describe("Admin Routes", () => {
         });
 
         expect(res.status).toBe(302); // Redirects back to dashboard
-        expect(res.headers.get("Location")).toBe("/admin?view=list");
+        expect(res.headers.get("Location")).toBe("/admin?view=list#your-feeds");
 
         // Verify feed was created in KV
         const feedList = (await mockEnv.EMAIL_STORAGE.get(