Fix SonoffLAN signing key: hardcode pre-computed HMAC key

The previous _compute_sign_key() function indexed into a base64 string
derived from the full SonoffLAN REGIONS dict (~243 entries). Our partial
dict only produced a 7876-char a string but needed index 7872+, so the
function must use the full dict.

Solution: pre-compute the key once from the full dict and hardcode the
resulting 32-byte ASCII key. This is deterministic — the SonoffLAN
algorithm always produces the same output regardless of when it runs.

The sonoff_ewelink driver now loads cleanly alongside all other drivers.

app/templates/sonoff/auth_settings.html

@@ -0,0 +1,104 @@
+{% extends "base.html" %}
+{% block title %}{{ board.name }} – eWeLink Settings{% endblock %}
+
+{% block content %}
+<div class="container py-4" style="max-width:560px">
+  <a href="{{ url_for('sonoff.gateway', board_id=board.id) }}" class="btn btn-outline-secondary btn-sm mb-3">
+    <i class="bi bi-arrow-left me-1"></i>Back to Gateway
+  </a>
+
+  <div class="card bg-dark border-secondary">
+    <div class="card-header">
+      <h5 class="mb-0"><i class="bi bi-key-fill me-2 text-warning"></i>eWeLink Account Settings</h5>
+    </div>
+    <div class="card-body">
+
+      {% with messages = get_flashed_messages(with_categories=true) %}
+        {% if messages %}
+          {% for cat, msg in messages %}
+            <div class="alert alert-{{ cat }} py-2 alert-dismissible fade show" role="alert">
+              {{ msg }}<button type="button" class="btn-close" data-bs-dismiss="alert"></button>
+            </div>
+          {% endfor %}
+        {% endif %}
+      {% endwith %}
+
+      <p class="text-muted small mb-4">
+        Enter your eWeLink account credentials. These are stored on the server and used
+        to authenticate with the eWeLink cloud API.
+        <strong>Tip:</strong> create a secondary eWeLink account and share your devices
+        with it to avoid being logged out of the mobile app.
+      </p>
+
+      <form method="post">
+        <div class="mb-3">
+          <label class="form-label">Email or Phone Number</label>
+          <input type="text" name="username" class="form-control"
+                 value="{{ board.config.get('ewelink_username', '') }}"
+                 placeholder="you@example.com" required>
+          <div class="form-text text-muted">Use email or full phone number (e.g. +1234567890)</div>
+        </div>
+
+        <div class="mb-3">
+          <label class="form-label">Password</label>
+          <div class="input-group">
+            <input type="password" name="password" id="pwdInput" class="form-control"
+                   placeholder="eWeLink password" required>
+            <button type="button" class="btn btn-outline-secondary" id="pwdToggle">
+              <i class="bi bi-eye" id="pwdIcon"></i>
+            </button>
+          </div>
+        </div>
+
+        <div class="mb-4">
+          <label class="form-label">Country / Region</label>
+          <select name="country_code" class="form-select">
+            {% for code, label in countries %}
+            <option value="{{ code }}" {% if code == current_country %}selected{% endif %}>
+              {{ label }}
+            </option>
+            {% endfor %}
+          </select>
+          <div class="form-text text-muted">
+            Determines which eWeLink server region to use (EU, US, AS, CN)
+          </div>
+        </div>
+
+        <div class="d-flex gap-2">
+          <button type="submit" class="btn btn-primary">
+            <i class="bi bi-check-lg me-1"></i>Save & Connect
+          </button>
+          <a href="{{ url_for('sonoff.gateway', board_id=board.id) }}" class="btn btn-outline-secondary">
+            Cancel
+          </a>
+        </div>
+      </form>
+
+      {% if board.config.get('ewelink_at') %}
+      <hr class="border-secondary mt-4">
+      <p class="small text-success mb-0">
+        <i class="bi bi-check-circle me-1"></i>
+        Currently connected to eWeLink
+        (region: <strong>{{ board.config.get('ewelink_region', '?') }}</strong>)
+      </p>
+      {% endif %}
+    </div>
+  </div>
+</div>
+{% endblock %}
+
+{% block scripts %}
+<script>
+document.getElementById('pwdToggle').addEventListener('click', function() {
+  const inp = document.getElementById('pwdInput');
+  const icon = document.getElementById('pwdIcon');
+  if (inp.type === 'password') {
+    inp.type = 'text';
+    icon.className = 'bi bi-eye-slash';
+  } else {
+    inp.type = 'password';
+    icon.className = 'bi bi-eye';
+  }
+});
+</script>
+{% endblock %}