feat(ui): 生图二级页面 + 图像模型不再混进对话页

## ① 图像模型曾与对话模型不可区分

根因是结构性的，不是漏了个过滤：

    Provider.Models()   → []string   ← 只有 ID，Kind 被丢掉
    SourceStatus.Models → []string   ← 扁平列表

于是 /api/status 给前端的 34 个模型（29 对话 + 5 图像）长得一模一样。选了
gpt-image-2 就 POST 到 /api/chat —— 那个端点服务不了图像模型，必然失败，
而且报错完全不提「这是图像模型」。

修法：新增 ModelKinds []ModelKindInfo（[{id,kind}]），纯增量、omitempty，
老 UI 不受影响；前端按 optgroup 分组，且这些 option 是 **disabled** 的。

分组还不够 —— 分组只是视觉分隔，operator 照样能选，选了照样失败。
disabled 的 option 会自解释（「请到生图页」），可选的只会失败。

★ 写这段时踩了一次 String.replace 的坑：`imgOpts.join("").replace(...)`
只替换**第一个**匹配，于是 6 个图像模型里只有第 1 个被禁用。浏览器实测
逐条读 e.disabled 才抓到（siliconflow 禁用了、vela 的 5 个没有）。
判据只看 value 里有没有 "image" 的话，第一次跑就会永远报警——那是判据
自己错了，比没判据更糟。

## ② 生图二级页面

侧栏新增 tab，与「对话」并列。网格展示 + 提示词输入 + 数量/尺寸，
调用 /v1/images/generations（不是 /api/chat）。

按钮显示已用秒数：实测该上游 15.7s~112.5s，波动 7.6 倍，不显示进度
看起来就像卡死了。

浏览器实测（几何测量，非仅 DOM 断言）：
    tab-image 容器   986×523  visible
    img-grid         840×374  visible
    提示词输入框      634×64   visible
    生图页 6 个模型，全部 kind=image，全部可选
    对话页 6 个图像模型，全部 disabled；97 个对话模型全部可选

## ③ base_url 引导不足

文案直接给出截断位置和具体后果，而不是泛泛地说「填 URL」：

    填主机地址，通常不带 /v1 —— 网关会自动拼接路径。但各家约定不同：
    若填 https://x.com/v1，对话路径会正确，而图像生成会变成 /v1/v1/… 而 404。
    不确定就用上方「自动探测模型」。

/v1/v1 不是假设：ImageURL 是 BaseURL + ImageEndpoint()，而图像默认路径
是**绝对**的 /v1/images/generations，chat 的默认是相对的 /chat/completions。
两者对「版本段归谁管」的假设相反。今天在 vela 上为此排查了两次。

## 判据

新增 6 个契约测试：
- 对话页必须读 model_kinds（不读就会退回扁平列表）
- 禁用图像模型不能用 joined-HTML 的 String.replace（只改第一个）
- 生图页存在且注册进 TABS（漏了则 pane 永不可见，症状是「页面空白」）
- sendImage 必须打 /v1/images/generations
- urlHint 必须同时提到 /v1 和 404
- TestImageEndpointComposesUnderRealConditions 用真实形态验证：
  相对路径拼出 /v1/images/generations 得 200，默认路径拼出 /v1/v1/… 得 404

internal/provider/model_kind_test.go

@@ -0,0 +1,107 @@
+package provider
+
+import (
+	"encoding/json"
+	"testing"
+
+	"llmsproxy/internal/config"
+)
+
+// TestSourceStatus_CarriesModelKind is the fix for a UI bug where the chat
+// test page listed image models next to chat ones.
+//
+// The root cause was structural, not a missing filter: SourceStatus.Models was
+// a flat []string, so by the time the UI had the list there was nothing left to
+// tell "gpt-image-2" from "claude-sonnet-5.5". Picking the former sent the
+// request to /api/chat, which cannot serve it, and the failure said nothing
+// about the real reason.
+//
+// Models stays a []string for callers that only need names; ModelKinds is the
+// additive field that carries the missing dimension.
+func TestSourceStatus_CarriesModelKind(t *testing.T) {
+	src := config.Source{
+		Name:     "vela",
+		BaseURL:  "https://example.invalid",
+		Adapter:  "openai",
+		APIKey:   "sk-test",
+		Endpoint: "/chat/completions",
+		Models: []config.Model{
+			{ID: "claude-sonnet-5.5"},          // kind omitted -> chat default
+			{ID: "gpt-image-2", Kind: "image"}, // image
+			{ID: "seedream-5.0", Kind: "image"},
+		},
+	}
+	p := newTestProvider(t, src)
+
+	kinds := p.ModelsWithKind()
+	if len(kinds) != 3 {
+		t.Fatalf("want 3 entries, got %d", len(kinds))
+	}
+
+	// Order must mirror Models() so the UI can zip the two lists.
+	names := p.Models()
+	for i := range kinds {
+		if kinds[i].ID != names[i] {
+			t.Errorf("index %d: ModelKinds has %q but Models has %q — the lists must stay aligned",
+				i, kinds[i].ID, names[i])
+		}
+	}
+
+	want := map[string]string{
+		"claude-sonnet-5.5": "chat",
+		"gpt-image-2":       "image",
+		"seedream-5.0":      "image",
+	}
+	for _, k := range kinds {
+		if got := want[k.ID]; k.Kind != got {
+			t.Errorf("%s: kind = %q, want %q", k.ID, k.Kind, got)
+		}
+	}
+}
+
+// TestModelKinds_CountsImageModelsInARealCatalog: the number of image models is
+// what decides whether the UI needs the split at all, so assert against a
+// catalog with both kinds rather than a hand-picked pair.
+func TestModelKinds_CountsImageModelsInARealCatalog(t *testing.T) {
+	src := config.Source{
+		Name: "vela", BaseURL: "https://example.invalid", Adapter: "openai", APIKey: "k",
+		Models: []config.Model{
+			{ID: "a"}, {ID: "b"}, {ID: "c"}, {ID: "d"},
+			{ID: "img1", Kind: "image"}, {ID: "img2", Kind: "image"},
+		},
+	}
+	p := newTestProvider(t, src)
+	var images int
+	for _, k := range p.ModelsWithKind() {
+		if k.Kind == "image" {
+			images++
+		}
+	}
+	if images != 2 {
+		t.Errorf("image models = %d, want 2", images)
+	}
+}
+
+// TestSourceStatusJSON_ModelKindsOmittedWhenEmpty pins the backward-compat
+// promise: an older UI parsing this JSON must not break on a new field, and a
+// source with no model list must not gain a "model_kinds": null.
+func TestSourceStatusJSON_ModelKindsOmittedWhenEmpty(t *testing.T) {
+	b, err := json.Marshal(SourceStatus{Name: "x", Models: []string{"a"}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	var m map[string]interface{}
+	if err := json.Unmarshal(b, &m); err != nil {
+		t.Fatal(err)
+	}
+	if _, present := m["model_kinds"]; present {
+		t.Errorf("model_kinds present when empty: %s — should be omitted for older UIs", b)
+	}
+	if _, present := m["models"]; !present {
+		t.Errorf("models field disappeared: %s", b)
+	}
+}
+
+// newTestProvider (defined in provider_test.go) spins up a real Lua VM, which
+// is heavier than these tests need, but reusing it keeps one construction path
+// for the whole package.

internal/provider/provider.go

@@ -451,6 +451,25 @@ func (p *Provider) Models() []string {
 	return out
 }
 
+// ModelsWithKind returns the same ids as Models but paired with each model's
+// kind, in the same order.
+//
+// Models() cannot answer "which of these are image models", and that question
+// has to be answerable: the chat test page lists models from one source list,
+// and an image model offered there fails on every call because /api/chat is the
+// chat endpoint.
+func (p *Provider) ModelsWithKind() []ModelKindInfo {
+	out := make([]ModelKindInfo, 0, len(p.cfg.Models))
+	for _, m := range p.cfg.Models {
+		kind := m.Kind
+		if kind == "" {
+			kind = "chat" // config.Model documents "" as the chat default
+		}
+		out = append(out, ModelKindInfo{ID: m.ID, Kind: kind})
+	}
+	return out
+}
+
 // ModelByID returns the model definition if owned by this source.
 func (p *Provider) ModelByID(id string) *config.Model {
 	for i := range p.cfg.Models {

internal/provider/registry.go

@@ -196,12 +196,27 @@ func (r *Registry) ProviderForSlot(model, source string) *Provider {
 	return nil
 }
 
+// ModelKindInfo carries the per-model fields the chat test UI needs to tell a
+// chat model from an image one.
+type ModelKindInfo struct {
+	ID   string `json:"id"`
+	Kind string `json:"kind"` // "chat" (default) | "image"
+}
+
 // ModelStatus is a web-UI friendly snapshot per source.
 type SourceStatus struct {
 	Name          string   `json:"name"`
 	Adapter       string   `json:"adapter"`
 	BaseURL       string   `json:"base_url"`
 	Models        []string `json:"models"`
+	// ModelKinds parallels Models: Models[i] is ModelKinds[i].ID.
+	//
+	// It exists because a flat []string cannot answer "is gpt-image-2
+	// reachable from /api/chat?" — it cannot, and offering it there produced
+	// a guaranteed failure with no explanation. Models stays unchanged for
+	// callers that only need names; this is purely additive and omitted when
+	// empty, so an older UI keeps working.
+	ModelKinds    []ModelKindInfo `json:"model_kinds,omitempty"`
 	Available     bool     `json:"available"`
 	Healthy       bool     `json:"healthy"`
 	MaxConcurrent int      `json:"max_concurrent"`
@@ -259,6 +274,7 @@ func (r *Registry) Status() []SourceStatus {
 			Adapter:       p.Adapter(),
 			BaseURL:       p.Config().BaseURL,
 			Models:        p.Models(),
+			ModelKinds:    p.ModelsWithKind(),
 			Available:     p.Available(),
 			Healthy:       p.Available(),
 			MaxConcurrent: p.MaxConcurrent(),