auth: replace password login with OIDC PKCE flow (Phase 5)

- Install expo-auth-session + expo-web-browser
- Add 'bincio-rec' URL scheme to app.json for deep-link redirect
- auth.ts: generate PKCE verifier/challenge, open bincio.org/oauth2/authorize
  in browser, exchange auth code for RS256 id_token, store in AsyncStorage
- SettingsScreen: remove handle/password fields, single 'Sign in with bincio'
  button that opens the browser flow

app.json

@@ -2,6 +2,7 @@
   "expo": {
     "name": "bincio-rec",
     "slug": "bincio-rec",
+    "scheme": "bincio-rec",
     "version": "1.0.0",
     "orientation": "portrait",
     "icon": "./assets/icon.png",
@@ -12,7 +13,9 @@
       "infoPlist": {
         "NSLocationWhenInUseUsageDescription": "bincio-rec uses your location to record your activity track.",
         "NSLocationAlwaysAndWhenInUseUsageDescription": "bincio-rec uses your location in the background to record your activity track.",
-        "UIBackgroundModes": ["location"]
+        "UIBackgroundModes": [
+          "location"
+        ]
       }
     },
     "android": {
@@ -39,7 +42,9 @@
     "web": {
       "favicon": "./assets/favicon.png"
     },
-    "ignorePaths": ["CLAUDE.md"],
+    "ignorePaths": [
+      "CLAUDE.md"
+    ],
     "plugins": [
       "expo-sqlite",
       [
@@ -56,7 +61,8 @@
           "icon": "./assets/icon.png",
           "color": "#3b82f6"
         }
-      ]
+      ],
+      "expo-web-browser"
     ]
   }
 }