[feat](trx-frontend-http): add guest button to auth gate

When RX access is unrestricted (no rx_passphrase required), show
a "Continue as Guest" button on the auth gate to allow immediate
access without requiring login. The button is only shown when guest
mode is available.

Guest mode:
- No authentication required
- Grants rx role immediately
- Can monitor radio but cannot transmit

Login path still available for full control access.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/index.html

@@ -35,6 +35,7 @@
         <input type="password" id="auth-passphrase" placeholder="Passphrase" autocomplete="off" style="width: 100%; padding: 0.5rem; margin-bottom: 1rem; border: 1px solid var(--border); border-radius: 0.25rem; background: var(--bg-secondary); color: var(--text); font-size: 1rem; box-sizing: border-box;" />
         <button type="submit" style="width: 100%; padding: 0.5rem; background: var(--accent-green); color: var(--bg-primary); border: none; border-radius: 0.25rem; font-weight: 600; cursor: pointer; font-size: 1rem; box-sizing: border-box;">Login</button>
       </form>
+      <button id="auth-guest-btn" type="button" style="width: 100%; padding: 0.5rem; background: var(--bg-secondary); color: var(--text); border: 1px solid var(--border); border-radius: 0.25rem; font-weight: 600; cursor: pointer; font-size: 1rem; box-sizing: border-box; margin-top: 0.5rem; display: none;">Continue as Guest</button>
       <div id="auth-error" style="color: #ff6b6b; font-size: 0.9rem; margin-top: 1rem; display: none;"></div>
       <div id="auth-role" style="margin-top: 1rem; color: var(--text-muted); font-size: 0.85rem; display: none;"></div>
     </div>