feat(desktop): add PreferencesAuthApprovalStore for persisted AUTH grants

Desktop persistence for the AuthApprovalPolicy in commons. Backs the
`auth_approvals` use case from the plan using java.util.prefs.Preferences
instead of the originally proposed sibling outbox.db SQLite table.

Trade-off rationale: the AUTH approval set per account is small
(typically < 50 relays for any user) and the read pattern is bounded
(one lookup per relay per session, easily cached in memory by the
policy layer). java.util.prefs is already in use elsewhere on desktop
(SearchHistoryStore, DesktopPreferences) and adds zero new
dependencies or schema migrations.

The retry_queue table from the same outbox.db proposal needs the
higher-throughput characteristics SQLite gives us; it remains scoped
to P3 (send visibility), which can introduce a proper sibling DB at
that point.

Per-account scoping by Preferences node — logout/account-delete calls
clear() which removeNode()s the subtree. ONCE scope is never written
to disk, enforced explicitly here in addition to the interface
contract.

Not yet wired into a DesktopAuthCoordinator (today desktop has NO
AUTH wiring at all). That wiring lands in P2.5 alongside the banner
UI.

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/auth/PreferencesAuthApprovalStore.kt

@@ -0,0 +1,81 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.amethyst.desktop.auth
+
+import com.vitorpamplona.amethyst.commons.relayClient.auth.AuthApprovalScope
+import com.vitorpamplona.amethyst.commons.relayClient.auth.AuthApprovalStore
+import com.vitorpamplona.quartz.nip01Core.relay.normalizer.NormalizedRelayUrl
+import java.util.prefs.Preferences
+
+/**
+ * Desktop persistence backend for [AuthApprovalStore] using
+ * `java.util.prefs.Preferences`.
+ *
+ * Trade-offs vs the full SQLite `auth_approvals` table proposed in the plan:
+ *
+ * - **Pro**: zero new dependencies, no schema migration, already proven for
+ *   other small desktop settings (per memory: `SearchHistoryStore`,
+ *   `DesktopPreferences`).
+ * - **Con**: flat key/value, no transactions, no native TTL. Acceptable here
+ *   because the approval set per account is small (≪50 relays for any user)
+ *   and the read pattern is "look up before signing AUTH" — once per relay
+ *   per session, easily cached in memory by the [AuthApprovalPolicy] layer.
+ *
+ * Per-account scoping is by Preferences node: each account gets its own node
+ * at `/com/vitorpamplona/amethyst/desktop/auth/<full-pubkey>/`. Logout calls
+ * [clear] which `removeNode()`s the per-account subtree.
+ *
+ * `ONCE` scope is never persisted — that's the in-memory contract enforced
+ * by the [AuthApprovalStore] interface. This implementation only writes
+ * `ALWAYS` and `BLOCKED`.
+ */
+class PreferencesAuthApprovalStore(
+    private val accountPubKeyHex: String,
+) : AuthApprovalStore {
+    private val node: Preferences =
+        Preferences.userRoot().node(
+            "/com/vitorpamplona/amethyst/desktop/auth/$accountPubKeyHex",
+        )
+
+    override suspend fun getScope(relayUrl: NormalizedRelayUrl): AuthApprovalScope? {
+        val raw = node.get(relayUrl.url, null) ?: return null
+        return runCatching { AuthApprovalScope.valueOf(raw) }.getOrNull()
+    }
+
+    override suspend fun setScope(
+        relayUrl: NormalizedRelayUrl,
+        scope: AuthApprovalScope,
+    ) {
+        if (scope == AuthApprovalScope.ONCE) {
+            // ONCE is the session-only contract from AuthApprovalStore — must
+            // not touch the persistent store, otherwise it would silently
+            // upgrade to "until next clear()".
+            return
+        }
+        node.put(relayUrl.url, scope.name)
+        node.flush()
+    }
+
+    override suspend fun clear() {
+        node.removeNode()
+        node.flush()
+    }
+}