feat: add Claude API bridge - transparent OpenAI-to-Anthropic protocol conversion

When provider is set to "claude" in config, all OpenAI-compatible API calls
are automatically bridged to Anthropic Claude Messages API, including:

- Non-streaming and streaming chat completions
- Tool calls (function calling) with full bidirectional conversion
- Eino multi-agent via HTTP transport hook (claudeRoundTripper)
- System message extraction, auth header conversion (Bearer → x-api-key)
- SSE stream format conversion (content_block_delta → OpenAI delta)
- TestOpenAI handler support for Claude connectivity testing

Zero impact when provider is "openai" or empty (default behavior unchanged).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/templates/index.html

@@ -1365,6 +1365,13 @@
                                 <div class="settings-subsection">
                                     <h4 data-i18n="settingsBasic.openaiConfig">OpenAI 配置</h4>
                                     <div class="settings-form">
+                                        <div class="form-group">
+                                            <label for="openai-provider">API 提供商</label>
+                                            <select id="openai-provider" style="width: 100%; padding: 0.5rem 0.75rem; border: 1px solid var(--border-color, #e2e8f0); border-radius: 6px; background: var(--card-bg, #fff); color: var(--text-color, #2d3748); font-size: 0.875rem;">
+                                                <option value="openai">OpenAI / 兼容 OpenAI 协议</option>
+                                                <option value="claude">Claude (Anthropic Messages API)</option>
+                                            </select>
+                                        </div>
                                         <div class="form-group">
                                             <label for="openai-base-url">Base URL <span style="color: red;">*</span></label>
                                             <input type="text" id="openai-base-url" data-i18n="settingsBasic.openaiBaseUrlPlaceholder" data-i18n-attr="placeholder" placeholder="https://api.openai.com/v1" required />