Allow building without UI (#1614)

Makefile

@@ -55,6 +55,7 @@ endif
 install:
 	go install $(KOPIA_BUILD_FLAGS) -tags "$(KOPIA_BUILD_TAGS)"
 
+install-noui: KOPIA_BUILD_TAGS=nohtmlui
 install-noui: install
 
 install-race:

internal/server/htmlui_embed.go

@@ -1,3 +1,6 @@
+//go:build !nohtmlui
+// +build !nohtmlui
+
 package server
 
 import (

internal/server/htmlui_fallback.go

@@ -0,0 +1,17 @@
+//go:build nohtmlui
+// +build nohtmlui
+
+package server
+
+import (
+	"embed"
+	"net/http"
+)
+
+//go:embed index.html
+var data embed.FS
+
+// AssetFile return a http.FileSystem instance that data backend by asset.
+func AssetFile() http.FileSystem {
+	return http.FS(data)
+}

internal/server/index.html

@@ -0,0 +1,11 @@
+<!DOCTYPE html>
+<html>
+    <head>
+        <title>Kopia UI not supported.</title>
+    </head>
+    <body>
+        <p>Kopia UI not supported, use an <a href="https://github.com/kopia/kopia/releases">official build</a>.</p>
+        <p>When compiling from source use:</p>
+        <pre>make html-ui && go install -tags embedhtml</pre>
+    </body>
+</html>