feat(app): Add confirmation modals for account deletion and social unlinking (#4729)

## Summary
- Added confirmation modals for both account deletion and social account
unlinking
- Both modals implement a two-step confirmation process with security
measures
- Users must type specific confirmation text to proceed with destructive
actions

## Changes

### Account Deletion Modal
- Created `DeleteAccountModal.svelte` component
- Users must type their account ID to confirm deletion
- 5-second countdown timer before final deletion
- Updated `AccountInfo.svelte` to use the modal

### Social Account Unlinking Modal  
- Created `UnlinkAccountModal.svelte` component
- Users must type "unlink {provider} and delete my xp" to confirm
- Same two-step flow and countdown timer as deletion modal
- Updated `SocialConnections.svelte` to use the modal instead of direct
unlinking
- Clear warnings about losing rewards and XP permanently