[CORE-002] Make accounts a first-class isolated runtime context

[veryCrunchy]

Roadmap

• Task ID: CORE-002
• Priority: P0

Problem

Users must be able to connect multiple Nextcloud accounts, including multiple identities on the same server, without state, cache, background work, or navigation leaking between them.

Acceptance criteria

• Multiple accounts can be added, switched, reauthenticated, and removed from a native account manager.
• Server URL plus login identity produces a stable non-personal account key; two users on one server remain distinct.
• Files, media, DAV, Talk, adaptive apps, caches, sync pairs, notifications, and background work are account-scoped.
• Account switching restores independent navigation and content state without restarting the app.
• Background notifications identify and open the correct account context.
• Removing one account requires confirmation and deletes only that account’s local credentials and state.
• Tests cover two accounts on one server, accounts on different servers, process restoration, and concurrent background work.