feat(ai): AI Data Access grants live on the node, not in localStorage

Operator: "the AI Data Access settings are not persistent through sessions,
often turns them all off."

They were stored in localStorage, which is scoped to an ORIGIN — and a node
answers on several: LAN address, Tailscale address, <host>.local, hostname.
Granting Media over the LAN and returning over Tailscale showed every switch
off again. Not reset: never set *there*. It also made a working content path
look broken, because every scope silently returns nothing without a grant, so
an ungranted permission is indistinguishable from an empty library — that is
exactly what an empty films search turned out to be.

The grant answers "what may the assistant read about THIS NODE", which is a
property of the node, not of one browser at one address. New
settings/ai_permissions.rs (same shape as session_policy: atomic temp+rename,
sanitised on read and write, fails closed on a corrupt file — an unreadable
grant file must never read as "everything allowed"). New ai.permissions.get /
.set, absent from the unauthenticated allowlist so they require a session.

Migration, not replacement: if this browser holds grants and the node holds
none, the local set is pushed UP rather than wiped. Without that, upgrading
would silently revoke the grants of everyone who set them before this change.
The node still wins in every other direction, so a revocation made on one
device takes effect everywhere — otherwise revoking would be impossible from a
second device.

Unknown category ids are stored verbatim rather than validated against a
hardcoded list: a third copy of that list would silently drop a new category on
upgrade. Storing a category grants nothing by itself — the broker checks before
fetching and the node re-checks before answering (T-13-33).

Hydration happens ONCE at broker start, not inside each permission gate: the
gates are hot-path, and awaiting there adds an RPC to every content and context
request. The first attempt did it per-gate and the existing broker tests caught
it by failing on consumed mocks.

Rust 7/7, store 18/18, broker 23/23.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

neode-ui/src/services/contextBroker.ts

@@ -139,6 +139,14 @@ export class ContextBroker {
   }
 
   start() {
+    // Grants live on the NODE, not in this browser. localStorage is per-origin
+    // and a node answers on several addresses (LAN, Tailscale, <host>.local),
+    // so a perfectly granted permission can read as denied at a second origin.
+    // Reconcile ONCE here rather than inside each permission gate: the gates
+    // are on the hot path, and an await there would add an RPC to every
+    // content and context request.
+    void useAIPermissionsStore().hydrate()
+
     this.listener = (e: MessageEvent) => this.handleMessage(e)
     window.addEventListener('message', this.listener)
   }