feat(aether-proxy): 网络层全面优化与代理耗时追踪

aether-proxy:
- 新增 20+ 可配置参数：Aether API/delegate/CONNECT 各阶段超时、连接池、TCP keepalive/nodelay
- AetherClient 支持指数退避重试(可配置次数/延迟)和 HTTP/2
- 新增 DNS 缓存(TTL + 容量限制)避免重复解析
- 新增并发连接数限制(Semaphore，默认基于硬件估算)
- CONNECT 隧道增加建连超时和升级超时
- TLS 增加握手超时、会话缓存(session ticket + memory cache)、ALPN 协商
- 新增 ProxyMetrics 收集请求计数和延迟，通过心跳上报
- delegate 响应注入 X-Proxy-Timing 头(dns_ms/upstream_ms/total_ms)

后端:
- StreamContext 和 handler 提取 X-Proxy-Timing 写入 proxy_info 追踪数据

前端:
- 请求时间线展示代理分阶段耗时(DNS/上游)
- Endpoint 添加按钮改为文字按钮样式

frontend/src/features/providers/components/EndpointFormDialog.vue

@@ -642,14 +642,13 @@
             </SelectContent>
           </Select>
           <Button
-            variant="ghost"
-            size="icon"
-            class="h-7 w-7 text-primary hover:text-primary"
-            title="添加"
+            variant="outline"
+            size="sm"
+            class="h-7 px-3"
             :disabled="!newEndpoint.api_format || (!newEndpoint.base_url?.trim() && !provider?.website?.trim()) || addingEndpoint"
             @click="handleAddEndpoint"
           >
-            <Plus class="w-3.5 h-3.5" />
+            添加
           </Button>
         </div>
         <!-- 卡片内容：URL 配置 -->

frontend/src/features/usage/components/HorizontalRequestTimeline.vue

@@ -213,6 +213,10 @@
                         v-if="currentAttempt.extra_data.proxy.ttfb_ms != null"
                         class="text-xs text-muted-foreground ml-1"
                       >{{ formatLatency(currentAttempt.extra_data.proxy.ttfb_ms) }}</span>
+                      <span
+                        v-if="currentAttempt.extra_data.proxy.timing"
+                        class="text-xs text-muted-foreground ml-1"
+                      >(DNS {{ formatLatency(currentAttempt.extra_data.proxy.timing.dns_ms) }} / 上游 {{ formatLatency(currentAttempt.extra_data.proxy.timing.upstream_ms) }})</span>
                     </span>
                   </div>
                   <div