fix: 节点离线时允许删除隧道关联，但禁止新增隧道 (#342)

- prepareTunnelCreateState: 更新隧道时允许已关联的离线节点，仅拒绝新增的离线节点
- syncForwardServicesWithWarnings: 离线节点跳过下发并返回警告，不再硬性失败
- applyTunnelRuntime: 所有节点类型均支持离线错误延迟处理
- 前端 validateTunnelForm: 编辑模式下跳过离线节点验证

Closes #342

plans/067-issue-342-offline-node-tunnel-edit.md

@@ -0,0 +1,22 @@
+# 067 - Issue #342: Allow Tunnel Edit with Offline Nodes
+
+**Issue:** https://github.com/Sagit-chu/flvx/issues/342
+
+## Problem
+When a node goes offline, users cannot edit tunnel configurations at all — including removing the faulty offline node. This creates a deadlock where users must wait for the offline node to recover or manually edit the database.
+
+## Changes Required
+
+### Backend
+
+- [x] 1. **`prepareTunnelCreateState`** (`mutations.go:2800`): Split the offline check into two modes:
+  - **Create (excludeTunnelID == 0)**: Keep current behavior — reject any offline non-remote node.
+  - **Update (excludeTunnelID > 0)**: Only reject **newly added** offline non-remote nodes. Allow existing offline nodes to remain (they'll be removed or kept). Query existing chain_tunnel records to determine which nodes are "old".
+
+- [x] 2. **`syncForwardServicesWithWarnings`** (`control_plane.go:231`): When a node is offline (sendNodeCommand fails with "节点不在线"), skip it and add a warning instead of returning a hard error. This allows forward rule modifications to succeed partially.
+
+- [x] 3. **`applyTunnelRuntime`** (`mutations.go:3190`): For non-remote local entry nodes, treat offline errors as deferrable (like remote nodes) so tunnel updates don't fail entirely when some nodes are offline.
+
+### Frontend
+
+- [x] 4. **`validateTunnelForm`** (`tunnel/form.ts`): Change validation to only block adding NEW offline nodes. When editing, offline nodes that are being removed should not block submission. Add isEdit parameter to distinguish create vs. edit.