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

app/admin/(protected)/medya/page.tsx

@@ -1,13 +1,15 @@
 import { PageHeader } from "@/components/admin/form";
-import { adminStorage, MEDIA_BUCKET_ID } from "@/lib/appwrite-server";
-import { Query } from "node-appwrite";
+import { MEDIA_BUCKET_ID, userStorage } from "@/lib/appwrite-server";
+import { requireSessionSecret } from "@/lib/auth";
+import { Query } from "appwrite";
 import { Upload } from "lucide-react";
 import { DeleteButton } from "@/components/admin/delete-button";
 import { uploadMedia, deleteMediaFile } from "@/lib/admin-actions";
 
 async function listFiles() {
   try {
-    const res = await adminStorage.listFiles({
+    const secret = await requireSessionSecret();
+    const res = await userStorage(secret).listFiles({
       bucketId: MEDIA_BUCKET_ID,
       queries: [Query.orderDesc("$createdAt"), Query.limit(100)],
     });

app/admin/(protected)/page.tsx

@@ -1,11 +1,13 @@
 import Link from "next/link";
 import { ArrowRight } from "lucide-react";
-import { adminDB, DATABASE_ID, TABLES } from "@/lib/appwrite-server";
-import { Query } from "node-appwrite";
+import { Query } from "appwrite";
+import { DATABASE_ID, TABLES, userDB } from "@/lib/appwrite-server";
+import { requireSessionSecret } from "@/lib/auth";
 
 async function safeCount(tableId: string, queries: string[] = []) {
   try {
-    const res = await adminDB.listRows({
+    const secret = await requireSessionSecret();
+    const res = await userDB(secret).listRows({
       databaseId: DATABASE_ID,
       tableId,
       queries: [...queries, Query.limit(1)],