fix: Node 26 uyumsuzluğunu çöz — node-appwrite -> appwrite SDK

Sorun:
- node-appwrite paketi 'node-fetch-native-with-agent' polyfill'i kullanıyor
- Node.js 26'nın undici implementation'ı ile uyumsuz
- 'fetch failed / InvalidArgumentError: invalid onError method' hatası
- Login dahil tüm Appwrite çağrıları başarısız

Çözüm:
- Tüm node-appwrite kullanımını browser SDK 'appwrite'a geçir
- Browser SDK native fetch kullanıyor, Node 26 uyumlu
- API key tabanlı admin client yerine session cookie tabanlı user client
- Public reads (read('any')): publicDB (auth'suz client)
- Admin CRUD: userDB(sessionSecret) (cookie'deki session)
- Storage upload doğrudan File objesi alıyor (InputFile.fromBuffer gerekmez)

Etkilenen dosyalar:
- lib/appwrite-server.ts: publicClient + sessionClient
- lib/auth.ts: requireSessionSecret eklendi
- lib/admin-actions.ts: tüm action'lar sessionClient kullanıyor
- app/actions.ts: publicDB
- lib/data.ts: publicDB
- app/admin/login/actions.ts: appwrite SDK
- app/admin/(protected)/page.tsx, medya/page.tsx: userDB/userStorage

End-to-end test edildi:
✓ Login (401 doğru hata)
✓ Public read (services)
✓ Anonim create (contact form)
✓ npm run build 23 route

lib/auth.ts

@@ -1,18 +1,20 @@
 import "server-only";
 import { cookies } from "next/headers";
 import { redirect } from "next/navigation";
-import { Account } from "node-appwrite";
-import { sessionClient } from "@/lib/appwrite-server";
+import { userAccount } from "@/lib/appwrite-server";
 
 export const SESSION_COOKIE = "kovak_session";
 
-export async function getCurrentUser() {
+export async function getSessionSecret() {
   const store = await cookies();
-  const secret = store.get(SESSION_COOKIE)?.value;
+  return store.get(SESSION_COOKIE)?.value ?? null;
+}
+
+export async function getCurrentUser() {
+  const secret = await getSessionSecret();
   if (!secret) return null;
   try {
-    const account = new Account(sessionClient(secret));
-    return await account.get();
+    return await userAccount(secret).get();
   } catch {
     return null;
   }
@@ -23,3 +25,9 @@ export async function requireUser() {
   if (!user) redirect("/admin/login");
   return user;
 }
+
+export async function requireSessionSecret() {
+  const secret = await getSessionSecret();
+  if (!secret) redirect("/admin/login");
+  return secret;
+}