New sidebar navigation replaces the top nav bar in the portal

Features

• New sidebar navigation replaces the top nav bar in the portal
• Mailbox list with creation flow in the sidebar
• Tenant switcher in the sidebar user menu
• Metrics page for deliverability stats
• Email detail side panel when viewing a mailbox
• Mailbox detail view with URL-driven tab bar (Inbox, Sent, Approvals, Flagged, Settings)
• Approval queue tab for reviewing pending emails per mailbox
• Sent tab with inbound/outbound direction filter
• Flagged tab showing policy-blocked and high-risk emails
• Emails now scoped to individual mailboxes across the platform
• Template types (marketing vs. transactional) with built-in compliance linting
• Multi-mailbox limits, attachment storage tracking, and quota notifications
• Payment failure grace period and auto-pause billing

Fixes

• Fixed Domain Connect private key handling and DKIM record matching
• Fixed blocked sends incorrectly counting toward the cooldown window
• Fixed 'Last 30 Days' deliverability stat showing lifetime totals
• Fixed malformed tenant cookie causing errors
• Fixed portal build crash and migration issues