feat(navigation): restore address book navigation in sidebar

[pagaille]

Fixes #3647

Summary

Address books used to be listed in the left sidebar (via the now-deleted SettingsSection.vue), allowing users to click on a book to filter their contacts. This was removed in bd536c0 when settings were migrated to a modal dialog, without any navigation replacement — making the "New address book" feature essentially invisible and unusable.

Changes:

• Add an "Address books" section in the sidebar listing all enabled address books with a contact counter
• Add ROUTE_ADDRESSBOOK constant and addressbook/:selectedAddressbook + addressbook/:selectedAddressbook/:selectedContact routes
• Add selectedAddressbook to RouterMixin
• Filter contactsList by the selected address book when on the addressbook route
• New contacts created while an address book is selected are saved into that address book (not always the default one)
• Fix Missing required param "selectedGroup" errors in selectFirstContactIfNone(), ContactsListItem, and ContactDetails (moveContactToAddressbook / copyContactToAddressbook) when navigating without a selectedGroup param

Test plan

• Create a new address book via Settings → it appears immediately in the left sidebar
• Click an address book in the sidebar → only its contacts are shown
• Click a contact in the filtered list → contact detail opens correctly
• Click "New contact" while an address book is selected → contact is saved in that address book
• Move/copy a contact to another address book → no routing error
• Disabling an address book via Settings → it disappears from the sidebar
• Existing group / circle / chart navigation is unaffected

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