Fix tag handling for multi-word tags

Tags with spaces (e.g. #science fiction) were breaking extraction because
TAG_RE only matched word characters — it would stop at the space and extract
#science instead of #science-fiction.

- TAG_RE (backend + frontend): add hyphens to character class so #science-fiction
  is recognized as a single tag: [\w][\w-]* per segment
- System prompt: instruct LLM to use hyphens in multi-word tags, never spaces
- tag_suggestions.py: update prompt example + sanitize output by replacing
  spaces with hyphens as a safety net regardless of LLM output
- append-tag route: sanitize incoming tag (spaces → hyphens) before appending

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/routes/notes.py

@@ -120,7 +120,7 @@ async def suggest_tags_route():
 async def append_tag_route(note_id: int):
     uid = get_current_user_id()
     data = await request.get_json()
-    tag = data.get("tag", "").strip()
+    tag = data.get("tag", "").strip().replace(" ", "-")
     if not tag:
         return jsonify({"error": "tag is required"}), 400