fix: add auth endpoints to server, fix auth bypass and registration

- server/: commit server code for the first time (was untracked)
  - POST /auth/signup and /auth/login endpoints now deployed
  - GET /v1/billing/summary now verifies user exists in auth_users
    (prevents stale JWTs from bypassing auth → fixes empty dashboard)
- app/_layout.tsx: dual-marker install check (SQLite + SecureStore)
  to detect fresh installs reliably on Android
- app/auth/login.tsx, signup.tsx: replace Ionicons leaf logo with
  actual app icon image (assets/icon.png)
- services/authService.ts: log HTTP status + server message on auth errors

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/package.json

@@ -0,0 +1,22 @@
+{
+  "name": "server",
+  "version": "1.0.0",
+  "description": "",
+  "main": "index.js",
+  "scripts": {
+    "start": "node index.js",
+    "rebuild:batches": "node scripts/rebuild-from-batches.js",
+    "diagnostics": "node scripts/plant-diagnostics.js",
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "keywords": [],
+  "author": "",
+  "license": "ISC",
+  "dependencies": {
+    "cors": "^2.8.6",
+    "dotenv": "^17.3.1",
+    "express": "^5.2.1",
+    "sqlite3": "^5.1.7",
+    "stripe": "^20.3.1"
+  }
+}