feat(arayüz): sayfa yönlendirme, korumalı sayfalar ve uygulama iskeleti

- react-router: Türkçe adresler (/defter, /plan, /birikim, ...), 404 sayfası
- Korumalı sayfalar: oturum yoksa /giris?donus=..., girişten sonra geri dönüş
- Açık yönlendirme koruması (safeReturnPath, 10 test)
- İskelet: mobilde 5 öğeli alt menü, masaüstünde sol kenar çubuğu
- Diğer sayfası (değerlendirme, raporlar, ayarlar, tema, çıkış)
- Arayüz metinleri lib/strings.ts'de toplanmaya başlandı
- Tasarım önizlemesi /dev/tasarim (yalnızca geliştirmede)
- Düzeltme: çıkışta önbellek temizliği ekranı güncellemiyordu

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/package-lock.json

@@ -17,6 +17,7 @@
         "radix-ui": "^1.8.1",
         "react": "^19.3.1",
         "react-dom": "^19.3.1",
+        "react-router": "^8.5.1",
         "shadcn": "^4.23.0",
         "tailwindcss": "^4.3.5",
         "tw-animate-css": "^1.4.0"
@@ -2682,6 +2683,12 @@
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       "license": "MIT"
     },
+    "node_modules/@remix-run/route-pattern": {
+      "version": "0.22.1",
+      "resolved": "https://registry.npmjs.org/@remix-run/route-pattern/-/route-pattern-0.22.1.tgz",
+      "integrity": "sha512-czdGJWh09Lapvcqt7Vb09KlrU2PhRJ8xQaI+AItTuD9aDJ5MAgxnS38lnys6Ll+6RJEqPiaUqTwIQhVLwFvv+w==",
+      "license": "MIT"
+    },
     "node_modules/@rolldown/binding-android-arm-eabi": {
       "version": "1.2.19",
       "resolved": "https://registry.npmjs.org/@rolldown/binding-android-arm-eabi/-/binding-android-arm-eabi-1.2.19.tgz",
@@ -4225,6 +4232,12 @@
       "integrity": "sha512-Kvp459HrV2FEJ1CAsi1Ku+MY3kasH19TFykTz2xWmMeq6bk2NU3XXvfJ+Q61m0xktWwt+1HSYf3JZsTms3aRJg==",
       "license": "MIT"
     },
+    "node_modules/cookie-es": {
+      "version": "3.1.1",
+      "resolved": "https://registry.npmjs.org/cookie-es/-/cookie-es-3.1.1.tgz",
+      "integrity": "sha512-UaXxwISYJPTr9hwQxMFYZ7kNhSXboMXP+Z3TRX6f1/NyaGPfuNUZOWP1pUEb75B2HjfklIYLVRfWiFZJyC6Npg==",
+      "license": "MIT"
+    },
     "node_modules/cross-spawn": {
       "version": "7.0.6",
       "resolved": "https://registry.npmjs.org/cross-spawn/-/cross-spawn-7.0.6.tgz",
@@ -6018,6 +6031,36 @@
         }
       }
     },
+    "node_modules/react-router": {
+      "version": "8.5.1",
+      "resolved": "https://registry.npmjs.org/react-router/-/react-router-8.5.1.tgz",
+      "integrity": "sha512-5u1eQ46QjuXjNfEnLwzrnou1Bm5dquO3xjVYkkMYCudtkicD2AQHtx+fvXzPATtPJx8TW6DR1x1TDRqsfHyZBw==",
+      "license": "MIT",
+      "dependencies": {
+        "@remix-run/route-pattern": "^0.22.1",
+        "cookie-es": "^3.1.1"
+      },
+      "engines": {
+        "node": ">=22.22.0"
+      },
+      "peerDependencies": {
+        "@types/react": ">=19.2.0",
+        "@types/react-dom": ">=19.2.0",
+        "react": ">=19.2.7",
+        "react-dom": ">=19.2.7"
+      },
+      "peerDependenciesMeta": {
+        "@types/react": {
+          "optional": true
+        },
+        "@types/react-dom": {
+          "optional": true
+        },
+        "react-dom": {
+          "optional": true
+        }
+      }
+    },
     "node_modules/react-style-singleton": {
       "version": "2.2.3",
       "resolved": "https://registry.npmjs.org/react-style-singleton/-/react-style-singleton-2.2.3.tgz",