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

.env.example

@@ -7,6 +7,9 @@ BASE_URL=http://localhost:8080
 # Database (SQLite, pure-Go modernc — no cgo)
 DATABASE_PATH=./data/petal.db
 
+# On-disk store for images pasted/dropped/inserted in the editor
+IMAGE_DIR=./data/images
+
 # LLM
 LLM_BACKEND=vllm                  # vllm | ollama
 LLM_ENDPOINT=http://localhost:8000   # vLLM :8000, Ollama :11434