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/lib/sqlite.js

@@ -0,0 +1,86 @@
+const fs = require('fs');
+const path = require('path');
+const sqlite3 = require('sqlite3').verbose();
+
+const getDefaultDbPath = () => {
+  return process.env.PLANT_DB_PATH || path.join(__dirname, '..', 'data', 'greenlns.sqlite');
+};
+
+const ensureDbDirectory = (dbPath) => {
+  const directory = path.dirname(dbPath);
+  fs.mkdirSync(directory, { recursive: true });
+};
+
+const openDatabase = (dbPath = getDefaultDbPath()) => {
+  ensureDbDirectory(dbPath);
+
+  return new Promise((resolve, reject) => {
+    const db = new sqlite3.Database(dbPath, (error) => {
+      if (error) {
+        reject(error);
+        return;
+      }
+      resolve(db);
+    });
+  });
+};
+
+const closeDatabase = (db) => {
+  return new Promise((resolve, reject) => {
+    db.close((error) => {
+      if (error) {
+        reject(error);
+        return;
+      }
+      resolve();
+    });
+  });
+};
+
+const run = (db, sql, params = []) => {
+  return new Promise((resolve, reject) => {
+    db.run(sql, params, function onRun(error) {
+      if (error) {
+        reject(error);
+        return;
+      }
+      resolve({
+        lastId: this.lastID,
+        changes: this.changes,
+      });
+    });
+  });
+};
+
+const get = (db, sql, params = []) => {
+  return new Promise((resolve, reject) => {
+    db.get(sql, params, (error, row) => {
+      if (error) {
+        reject(error);
+        return;
+      }
+      resolve(row || null);
+    });
+  });
+};
+
+const all = (db, sql, params = []) => {
+  return new Promise((resolve, reject) => {
+    db.all(sql, params, (error, rows) => {
+      if (error) {
+        reject(error);
+        return;
+      }
+      resolve(rows || []);
+    });
+  });
+};
+
+module.exports = {
+  all,
+  closeDatabase,
+  get,
+  getDefaultDbPath,
+  openDatabase,
+  run,
+};