feat(kol): add selection-based AI optimize for prompt editor

Trigger an AI optimize panel from a text selection in the KOL prompt
editor, accept a user instruction, and stream a preview before replacing
the selected fragment. Extend the shared editor AI assist panel with
boundary-aware placement so it can be hosted inside scoped surfaces, and
update the backend optimize prompt to honor the user instruction.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/internal/tenant/app/kol_assist_runtime_test.go

@@ -72,6 +72,20 @@ func TestParseKolAssistResponseSupportsStringVariables(t *testing.T) {
 	require.Equal(t, "城市", result.Schema.Variables[0].Key)
 }
 
+func TestBuildKolAssistUserPromptIncludesOptimizeInstruction(t *testing.T) {
+	t.Parallel()
+
+	prompt, err := BuildKolAssistUserPrompt(AssistRequest{
+		Mode:           kolAssistModeOptimize,
+		Description:    "让表达更清晰，但保留变量",
+		CurrentContent: "围绕 {{城市}} 写一段内容",
+	})
+
+	require.NoError(t, err)
+	require.Contains(t, prompt, "Instruction:\n让表达更清晰，但保留变量")
+	require.Contains(t, prompt, "PROMPT:\n围绕 {{城市}} 写一段内容")
+}
+
 func TestExtractKolAssistContentFromPartialJSON(t *testing.T) {
 	t.Parallel()