Editor: font-size presets + image insert/export support

Two editor features that were in flight alongside the sound work:
- FontSize TipTap extension (rides on textStyle) with Small/Normal/Large/
  Title presets in the toolbar; StatusBar + CSS support
- Image handling: internal/images handler, upload route + config, client
  API, EditorCore wiring, and md/html/docx export support for images

Claude-Session: https://claude.ai/code/session_016Yr6jELuRc7hyzYLccQKZd

internal/config/config.go

@@ -11,6 +11,7 @@ type Config struct {
 	Port         string
 	BaseURL      string
 	DatabasePath string
+	ImageDir     string // on-disk store for editor image uploads
 
 	// LLM
 	LLMBackend   string // "vllm" | "ollama"
@@ -32,6 +33,7 @@ func Load() *Config {
 		Port:         env("PORT", "8080"),
 		BaseURL:      env("BASE_URL", "http://localhost:8080"),
 		DatabasePath: env("DATABASE_PATH", "./data/petal.db"),
+		ImageDir:     env("IMAGE_DIR", "./data/images"),
 
 		LLMBackend:   env("LLM_BACKEND", "vllm"),
 		LLMEndpoint:  env("LLM_ENDPOINT", "http://localhost:8000"),