More detailed card tooltips

Card tooltips now show what a card represents *as* an Obstacle (the
rulebook obstacle table entry) in addition to its suit theme, which is
relevant when challenging another Pi-Rat. The obstacle tables are served
from the backend via GET /api/obstacles (single source of truth in
cards.py) and cached client-side in cards.js, with a retry so a transient
first-load failure doesn't silently disable the tooltips.

Enriched everywhere cards appear in the challenge flow: hand/board cards
(Card.svelte), PvP defend buttons and the temporary-obstacle line
(ChallengePanel), and the duel card selector, which also shows an
inline description of what the chosen card becomes as an Obstacle.

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

frontend/src/main.js

@@ -10,6 +10,10 @@ import '@fontsource/alegreya-sans/700.css'
 
 import './app.css'
 import App from './App.svelte'
+import { loadObstacleTable } from './lib/cards'
+
+// Warm the obstacle-table cache so card tooltips can describe cards as Obstacles
+loadObstacleTable()
 
 const app = mount(App, {
   target: document.getElementById('app'),