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>

frontend/src/components/TranscriptPane.tsx

@@ -1,6 +1,7 @@
 import { useEffect, useRef } from "react";
 
 import { Message } from "../api/games";
+import MarkdownContent from "./MarkdownContent";
 
 function senderLabel(message: Message): string {
   if (message.sender_type === "dm") return "Dungeon Master";
@@ -36,8 +37,8 @@ export default function TranscriptPane({
             }
           >
             {senderLabel(m)}:
-          </span>{" "}
-          <span className="whitespace-pre-wrap">{m.content}</span>
+          </span>
+          <MarkdownContent content={m.content} />
         </div>
       ))}
       {typing && (