Add character detail, markdown rendering, wider layout, and LLM provider switch

- Character sheets gain structured combat_stats, abilities (name +
  plain-language explanation), and equipment fields, populated by the
  upsert_character_sheet tool; the detail page renders them as their
  own sections instead of one prose block, with a player-card-style
  attribute grid (full ability names + modifiers).
- Fix double-escaped unicode occasionally left in tool-call JSON
  arguments (a model quirk) and stop the app's own json.dumps calls
  from re-introducing it (ensure_ascii=False).
- Render chat messages as markdown (react-markdown + Tailwind
  typography) instead of raw text, and widen the games/characters/
  chat layout to use more of the screen on desktop.
- Sharpen the DM system prompt: state the ability + DC for each
  offered action option before the player commits, and narrate rolls
  as DC → result → pass/fail before the story consequence; also stop
  re-asking Session-0 questions already answered in the game's
  name/description.
- Make the DM's LLM provider configurable (xai default, lmstudio for
  a local OpenAI-compatible server via host.docker.internal) instead
  of hardcoded to x.ai.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/config.py

@@ -6,10 +6,19 @@ class Settings(BaseSettings):
 
     database_url: str = "postgresql+asyncpg://dnd:changeme@postgres:5432/dnd"
 
+    # "xai" (default, cloud) or "lmstudio" (local, OpenAI-compatible server)
+    llm_provider: str = "xai"
+
     xai_api_key: str = ""
     xai_base_url: str = "https://api.x.ai/v1"
     dm_model: str = "grok-4.6"
 
+    # host.docker.internal, not a LAN IP — the backend runs inside Docker Desktop's own
+    # network namespace, which can reach the Windows host via this name but not necessarily
+    # via whatever IP the host itself sees for the machine (e.g. a WSL2-internal address).
+    lmstudio_base_url: str = "http://host.docker.internal:1234/v1"
+    lmstudio_api_key: str = ""
+
     jwt_secret: str = "dev-insecure-secret-change-me"
     reset_password_secret: str = "dev-insecure-secret-change-me"
     verification_token_secret: str = "dev-insecure-secret-change-me"