Fix Bedrock advocate synthesis: engine guard, response_format, Claude engine option

- DeepResearchAgent: engine guard now accepts dbn_legal_v3, claude_sonnet, claude_haiku
  (previously stripped these to azure_mini, breaking dbn_legal_v3 selection and
  preventing Claude engines from reaching the correct synthesis branch)
- DbnBedrockGateway: remove response_format=json_object from chat payload — LiteLLM
  converts this to a tool-use constraint for Bedrock, routing output into tool_calls
  instead of content (root cause of the {} empty brief)
- advocate.php: add Claude Sonnet 4.6 (AWS Bedrock) engine option
- account, billing, dashboard, nav, min-sak: pending UI/flow changes from prior sessions

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

includes/nav.php

@@ -88,7 +88,7 @@ $_navAssetBase = str_contains($_navScriptPath, '/dashboard/') ? '../assets' : 'a
             <?= htmlspecialchars(dbnToolsT('nav_login', $_navLang)) ?>
         </a>
 <?php else: ?>
-        <a class="dbn-nav__account-link" href="/dashboard.php#account" title="<?= htmlspecialchars($_navEmail) ?>">
+        <a class="dbn-nav__account-link" href="/account.php" title="<?= htmlspecialchars($_navEmail) ?>">
 <?php if ($_navUser !== ''): ?>
             <span class="dbn-nav__username"><?= htmlspecialchars($_navUser) ?></span>
 <?php endif; ?>