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/scripts/plant-diagnostics.js

@@ -0,0 +1,23 @@
+#!/usr/bin/env node
+/* eslint-disable no-console */
+require('dotenv').config();
+
+const { closeDatabase, openDatabase } = require('../lib/sqlite');
+const { ensurePlantSchema, getPlantDiagnostics } = require('../lib/plants');
+
+const main = async () => {
+  const db = await openDatabase();
+  try {
+    await ensurePlantSchema(db);
+    const diagnostics = await getPlantDiagnostics(db);
+    console.log(JSON.stringify(diagnostics, null, 2));
+  } finally {
+    await closeDatabase(db);
+  }
+};
+
+main().catch((error) => {
+  console.error('Failed to read plant diagnostics.');
+  console.error(error instanceof Error ? error.stack || error.message : String(error));
+  process.exit(1);
+});