feat(server): add localized agent starter config endpoint

Add a public GET /api/ui/agent-starters endpoint returning localized
empty-state content (title, description, preview images, and starter
prompts) for the canvas Agent panel. Content is server-configurable via
AgentStarter config with built-in zh-CN/en-US fallbacks, and the route is
exempt from authentication so shared canvases render the same content.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/logic/get_agent_starter_config_logic_test.go

@@ -0,0 +1,70 @@
+package logic
+
+import (
+	"context"
+	"testing"
+
+	"img_infinite_canvas/internal/config"
+	"img_infinite_canvas/internal/svc"
+	"img_infinite_canvas/internal/types"
+)
+
+func TestGetAgentStarterConfigReturnsLocalizedDefaults(t *testing.T) {
+	logic := NewGetAgentStarterConfigLogic(context.Background(), &svc.ServiceContext{})
+
+	response, err := logic.GetAgentStarterConfig(&types.GetAgentStarterConfigRequest{Locale: "en-US"})
+	if err != nil {
+		t.Fatalf("get agent starter config: %v", err)
+	}
+	if response.Title != "What would you like to create today?" {
+		t.Fatalf("unexpected title: %q", response.Title)
+	}
+	if len(response.PreviewImages) != 3 {
+		t.Fatalf("expected three preview images, got %d", len(response.PreviewImages))
+	}
+	if len(response.Starters) != 5 {
+		t.Fatalf("expected five starters, got %d", len(response.Starters))
+	}
+	expected := []types.AgentStarterItem{
+		{Id: "ecommerce", Label: "E-commerce product", Prompt: "Create a premium ecommerce product image for a portable espresso machine, with realistic lighting, clean composition, benefit callouts, and marketplace-ready visual hierarchy"},
+		{Id: "poster", Label: "Poster", Prompt: "Design a bold launch poster for a new skincare product line, with strong typography, product focus, campaign headline, date, and a polished premium layout"},
+		{Id: "brand-visual", Label: "Brand visual", Prompt: "Create a brand identity direction for a modern wellness drink brand, including color palette, typography mood, logo usage, packaging accents, and social-ready visual elements"},
+		{Id: "social-media", Label: "Social media", Prompt: "Create a square Instagram promotion for a weekend fashion sale, with product cutouts, discount headline, brand colors, and a layout that works as a paid social ad"},
+		{Id: "logo", Label: "Logo design", Prompt: "Design a clean vector logo for a direct-to-consumer home goods brand named Loom & Line, with a simple mark, balanced wordmark, and clear black-and-white usage"},
+	}
+	for index, item := range expected {
+		if response.Starters[index] != item {
+			t.Fatalf("unexpected starter %d: got %#v, want %#v", index, response.Starters[index], item)
+		}
+	}
+}
+
+func TestGetAgentStarterConfigUsesConfiguredContent(t *testing.T) {
+	serviceContext := &svc.ServiceContext{Config: config.Config{AgentStarter: config.AgentStarterConfig{
+		PreviewImages: []string{" https://cdn.example.com/one.webp ", ""},
+		ZhCN: config.AgentStarterLocaleConfig{
+			Title:       "新的入口标题",
+			Description: "新的入口说明",
+			Starters: []config.AgentStarterItemConfig{
+				{ID: "campaign", Label: "运营活动", Prompt: "Create a campaign visual"},
+				{ID: "campaign", Label: "重复项", Prompt: "Ignore duplicate"},
+				{ID: "incomplete", Label: "不完整"},
+			},
+		},
+	}}}
+	logic := NewGetAgentStarterConfigLogic(context.Background(), serviceContext)
+
+	response, err := logic.GetAgentStarterConfig(&types.GetAgentStarterConfigRequest{Locale: "zh-CN"})
+	if err != nil {
+		t.Fatalf("get configured agent starter content: %v", err)
+	}
+	if response.Title != "新的入口标题" || response.Description != "新的入口说明" {
+		t.Fatalf("unexpected configured copy: %+v", response)
+	}
+	if len(response.PreviewImages) != 1 || response.PreviewImages[0] != "https://cdn.example.com/one.webp" {
+		t.Fatalf("unexpected configured images: %#v", response.PreviewImages)
+	}
+	if len(response.Starters) != 1 || response.Starters[0].Id != "campaign" {
+		t.Fatalf("unexpected configured starters: %#v", response.Starters)
+	}
+}