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/pages/ResetPassword.tsx

@@ -25,7 +25,7 @@ export default function ResetPassword() {
 
   if (!token) {
     return (
-      <div className="mx-auto mt-16 max-w-sm px-4 text-slate-300">
+      <div className="mx-auto mt-16 max-w-md px-4 text-slate-300">
         Kein gültiger Reset-Link.{" "}
         <Link to="/forgot-password" className="text-amber-400">
           Neu anfordern
@@ -35,7 +35,7 @@ export default function ResetPassword() {
   }
 
   return (
-    <div className="mx-auto mt-16 max-w-sm px-4">
+    <div className="mx-auto mt-16 max-w-md px-4">
       <h1 className="mb-6 text-2xl font-bold text-amber-400">Neues Passwort setzen</h1>
       {done ? (
         <p className="text-slate-300">Passwort geändert. Du wirst weitergeleitet…</p>