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/services/llm.py

@@ -337,6 +337,7 @@ async def build_context(
         tool_lines.append("When the user says 'remind me' with a time before an event, use the reminder_minutes parameter.")
         tool_lines.append("Use search_todos to find a specific CalDAV todo by keyword when list_todos would return too many results.")
     tool_lines.append("For relative dates like 'Friday' or 'next week', resolve them to YYYY-MM-DD format.")
+    tool_lines.append("When writing #tags in note bodies, use hyphens for multi-word tags (e.g. #science-fiction, #space-travel). Never use spaces inside a tag.")
     tool_lines.append(
         "Use update_note to edit/expand an existing note OR to update a task's status/priority/due_date. "
         "Use create_note ONLY for genuinely new notes with a different title. "