v0.4.0 — Spend Alerts & Billing Models

New

• 💰 Spend alerts: badge animation (🌕/💰) + Chrome notification on spending
• 📊 Spend prediction: days remaining shown next to balance
• ▲▼ Trend indicator: balance up/down/flat
• 🆕 xAI / Grok + Perplexity providers
• 📐 Multi-billing-model support: prepaid / usage / quota
• ⭐ Default visibility: mainstream providers shown by default
• 🔔 Sound toggle in Settings (animation always on)

Test Coverage

• 11 test files, 72 tests passed
• spend-checker, new providers, balanceType, popular markers