feat: bet placement panel with odds and payout preview

BetPanel component with pick-winner buttons, preset amounts (21/100/500/
1000 sats), Cashu token paste, odds display from server, and payout
preview with 5% house cut. Wired into FightCardPage for scheduled fights.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/FightCardPage.vue

@@ -3,6 +3,7 @@ import { ref, computed, onMounted, onUnmounted, watch } from 'vue'
 import { RouterLink } from 'vue-router'
 import SpritePreview from '../components/SpritePreview.vue'
 import PixelGlove from '../components/PixelGlove.vue'
+import BetPanel from '../components/BetPanel.vue'
 
 interface BotInfo {
   name: string
@@ -15,6 +16,8 @@ interface BotInfo {
 
 interface UpcomingFight {
   id: string
+  botAId: string
+  botBId: string
   botA: BotInfo | null
   botB: BotInfo | null
   arenaInfo: { name: string } | null
@@ -409,6 +412,15 @@ onUnmounted(() => {
             </button>
           </div>
 
+          <!-- Bet panel for upcoming fights -->
+          <div v-if="featured && featured.botA && featured.botB && featured.status === 'scheduled'" class="px-4 pb-2 shrink-0">
+            <BetPanel
+              :fight-id="featured.id"
+              :bot-a="{ id: featured.botAId, name: featured.botA.name, eloRating: featured.botA.eloRating, tier: featured.botA.tier }"
+              :bot-b="{ id: featured.botBId, name: featured.botB.name, eloRating: featured.botB.eloRating, tier: featured.botB.tier }"
+            />
+          </div>
+
           <!-- Enter the ring -->
           <div class="px-4 pb-3 lg:pb-5 shrink-0">
             <RouterLink