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/llm/orchestrator.py

@@ -7,6 +7,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from app.config import settings
 from app.llm.client import get_dm_system_prompt, get_llm_client
 from app.llm.context import build_context
+from app.llm.json_utils import fix_double_escaped_unicode
 from app.llm.tools import character_sheet, dice
 from app.models.game import Game
 from app.models.message import Message
@@ -81,13 +82,13 @@ async def run_dm_turn(
         messages.append(message.model_dump(exclude_unset=True))
 
         for tool_call in message.tool_calls:
-            tool_input = json.loads(tool_call.function.arguments)
+            tool_input = fix_double_escaped_unicode(json.loads(tool_call.function.arguments))
             result = await _execute_tool_call(session, game_id, tool_call.function.name, tool_input)
             messages.append(
                 {
                     "role": "tool",
                     "tool_call_id": tool_call.id,
-                    "content": json.dumps(result),
+                    "content": json.dumps(result, ensure_ascii=False),
                 }
             )
     else: