feat(web): add Space switcher and shared channel categories

[eminogrande]

Problem

Buzz's existing channel sections are personal sidebar organization, not shared Discord-style categories. The Nuri web client also has no Space rail or Create/Join onboarding.

User flow

1. chat.nuri.com shows the user's joined Spaces in a left rail and offers Create Space / Join Space.
2. Selecting a Space connects to its isolated <slug>.relay.nuri.com tenant.
3. Inside the Space, shared Categories group stream/forum Channels; #general opens after creation or join.

Acceptance criteria

1. Add shared category metadata and channel assignments scoped to one community, owner/admin-writeable and member-readable, with ordering and deletion semantics that never move channels between communities.
2. Add a browser Space rail and Create/Join dialogs backed by the self-service Space APIs; switching Spaces tears down subscriptions and reloads only the selected tenant while preserving the in-memory Nuri signer.
3. Render Categories → Channels → history/live subscription/composer with loading, empty, reconnect and permission states; Playwright covers create → switch → join → send/receive across two identities.

Not doing

• Personal section sync parity
• Voice/huddles, files, reactions or threads in the first web release
• Categories nested inside categories

Branch reference

Integration follows feat/nuri-web-chat and the self-service Spaces issue.

Session

@session:default/20260724_150854_84a994