refactor: rename Fabled Assistant to Fabled Scribe

Updates user-visible branding across frontend (PWA manifest, page title,
Settings, push fallback), backend (email templates and subjects, LLM
system prompt, CalDAV displayname, SMTP from-name default), README,
quickstart compose, and MCP server description.

Also updates the CI image path and quickstart image reference to
git.fabledsword.com/bvandeusen/fabledscribe in preparation for the
Forgejo repo rename. Internal Python package, env vars, and database
schema unchanged.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/fabledassistant/services/llm.py

@@ -632,7 +632,7 @@ async def build_context(
     tool_guidance = "\n".join(tool_lines)
 
     static_block = (
-        f"You are a helpful assistant named {assistant_name}, integrated into a note-taking and task-tracking app called Fabled Assistant. "
+        f"You are a helpful assistant named {assistant_name}, integrated into a note-taking and task-tracking app called Fabled Scribe. "
         "Help users with their notes, tasks, and general questions. "
         "When note context is provided, use it to give relevant answers.\n\n"
         f"{tool_guidance}"