feat: human vs AI mode — live typing challenges, baby growth system, SSE rounds

- Add choose-mode step: "I BUILD BOTS" vs "I FIGHT MYSELF" paths
- Human registration with baby avatar picker, no webhook required
- Live fight scene with SSE round streaming and real-time challenge UI
- 5-second timer per round, submit answers via browser
- Baby → toddler → kid → teen → adult → hero → super growth stages
- Huge sparkly baby eyes, diapers, pacifiers, bibs, rattles, rosy cheeks
- Speech bubble positioning fix (pushed to outside of sprite)
- Canvas text rendering via offscreen canvas to bypass kaplay color issues
- Voice timing improvements: await pauses between voice lines and hits
- 30 devastating announcement lines, 15 critical/hit word variants
- Orchestrator human player detection + waitForHumanResponse system
- Server endpoints: GET /challenge/:botId, POST /respond/:botId
- Human player auth: register-human route, isHuman flag on login

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

frontend/src/composables/useNostr.ts

@@ -16,6 +16,7 @@ interface BotData {
   archetype: string
   profilePicUrl: string | null
   customization: BotCustomization | null
+  isHuman?: boolean
   eloRating: number
   wins: number
   losses: number
@@ -169,6 +170,43 @@ export function useNostr() {
     }
   }
 
+  async function registerHuman(name: string, avatarSeed?: string): Promise<BotData> {
+    if (!pubkey.value) throw new Error('Not logged in')
+
+    const res = await fetch('/api/auth/register-human', {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({
+        pubkey: pubkey.value,
+        name,
+        avatarSeed,
+        profilePicUrl: profilePicUrl.value,
+      }),
+    })
+
+    const data = await res.json()
+    if (!res.ok) throw new Error(data.error || 'Registration failed')
+
+    bot.value = {
+      id: data.id,
+      name: data.name,
+      avatarSeed: avatarSeed || data.name,
+      archetype: 'human',
+      profilePicUrl: profilePicUrl.value,
+      customization: null,
+      isHuman: true,
+      eloRating: 1200,
+      wins: 0,
+      losses: 0,
+      winStreak: 0,
+      bestStreak: 0,
+      tier: 0,
+    }
+    store('bf_bot', bot.value)
+
+    return bot.value
+  }
+
   function logout() {
     pubkey.value = null
     bot.value = null
@@ -187,6 +225,7 @@ export function useNostr() {
     hasExtension,
     login,
     registerBot,
+    registerHuman,
     updateCustomization,
     logout,
   }