feat: installment panel, notification center, comments, address book

[NnamdiCyber]

Summary

Implements four high-complexity issues in a single PR.

Stellar-split#39 Add payment installment UI

• New InstallmentPanel component on the invoice detail page
• Calls splitClient.getInstallmentPlan(invoiceId, publicKey)
• Timeline of (date, amount) pairs; next due installment highlighted in indigo; past/paid installments dimmed with ✓ badge
• Shows "No plan registered" when no plan exists
• Only rendered when a wallet is connected

Stellar-split#30 Build invoice notification center

• New NotificationCenter component added to the global layout header
• Bell icon with unread count badge (capped at 9+)
• Dropdown lists notifications in reverse-chronological order from localStorage key stellarsplit_notifications
• Unread notifications have a blue dot indicator
• "Mark all as read" clears the badge; clicking a notification navigates to the relevant invoice
• Listens for storage events to pick up notifications written by the polling mechanism

Stellar-split#43 Add invoice comment system

• New CommentSection component at the bottom of the invoice detail page
• Stores { id, invoiceId, walletAddress, text, timestamp }[] in localStorage key stellarsplit_comments
• Filtered by both invoice ID and wallet address — private per wallet
• Add and delete individual comments; timestamps shown in relative format ("2 hours ago")
• Only rendered when a wallet is connected

Stellar-split#41 Implement address book

• New src/lib/addressBook.ts with getAddressBook, addEntry, updateEntry, removeEntry, searchEntries (max 50 entries, localStorage key stellarsplit_address_book)
• RecipientForm updated with auto-suggest dropdown (triggers at ≥2 chars, matches nickname or address prefix); valid G... addresses auto-saved on blur
• New management page at /address-book with add/edit/delete UI
• "Address Book" link added to the global header

Testing

• npm run build passes with no TypeScript errors
• All pages render; /address-book is a new static route