feat(settings): add home content customization and config injection

- Add home_content setting for custom homepage (HTML or iframe URL)
- Inject public settings into index.html to eliminate page flash
- Support ETag caching with automatic invalidation on settings update
- Add Vite plugin for dev mode settings injection
- Refactor HomeView to use appStore instead of local API calls

backend/internal/web/embed_off.go

@@ -4,11 +4,38 @@
 package web
 
 import (
+	"context"
+	"errors"
 	"net/http"
 
 	"github.com/gin-gonic/gin"
 )
 
+// PublicSettingsProvider is an interface to fetch public settings
+// This stub is needed for compilation when frontend is not embedded
+type PublicSettingsProvider interface {
+	GetPublicSettingsForInjection(ctx context.Context) (interface{}, error)
+}
+
+// FrontendServer is a stub for non-embed builds
+type FrontendServer struct{}
+
+// NewFrontendServer returns an error when frontend is not embedded
+func NewFrontendServer(settingsProvider PublicSettingsProvider) (*FrontendServer, error) {
+	return nil, errors.New("frontend not embedded")
+}
+
+// InvalidateCache is a no-op for non-embed builds
+func (s *FrontendServer) InvalidateCache() {}
+
+// Middleware returns a handler that returns 404 for non-embed builds
+func (s *FrontendServer) Middleware() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		c.String(http.StatusNotFound, "Frontend not embedded. Build with -tags embed to include frontend.")
+		c.Abort()
+	}
+}
+
 func ServeEmbeddedFrontend() gin.HandlerFunc {
 	return func(c *gin.Context) {
 		c.String(http.StatusNotFound, "Frontend not embedded. Build with -tags embed to include frontend.")