feat(desktop): add custom feeds system with creation, pinning, and inline switching

- FeedDefinition data model with FeedSource sealed interface (Filter, Following, Global, DVM, PeopleList, InterestSet, SingleRelay)
- FeedDefinitionRepository with StateFlow, groupedFeeds, pin/unpin/reorder (max 3 pinned)
- JSON serialization via Jackson with round-trip tests (18 unit tests)
- SearchQuery.toFeedDefinition() bridge for creating feeds from search
- FeedBuilderDialog with author search (cache lookup + npub decode), hashtag/relay inputs, kind filter checkboxes, exclude authors/keywords, Cmd+S save, Enter to add
- FeedsDrawerTab in app drawer with edit/delete/pin/unpin actions
- Feeds tab in top header (FilterChips) with inline mode switching
- CustomFeedScreen + DesktopCustomFeedFilter + createCustomFeedSubscription for relay-based custom feed content
- FeedDefinitionEvent (kind 31890) in quartz for future publish/import
- Local persistence via java.util.prefs.Preferences (auto-save on change)
- DeckColumnType.CustomFeed for navigation integration
- Renamed Home to Feeds in nav rail
- Fix: AnimatedGifImage race condition (coerceIn on frame index)
- Fix: search results margins (sidePadding applied consistently)
- Fix: app drawer search includes feeds in results

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/feedDefinition/FeedDefinitionEvent.kt

@@ -0,0 +1,53 @@
+/*
+ * 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.quartz.feedDefinition
+
+import androidx.compose.runtime.Immutable
+import com.vitorpamplona.quartz.nip01Core.core.BaseAddressableEvent
+import com.vitorpamplona.quartz.nip01Core.core.HexKey
+import com.vitorpamplona.quartz.nip01Core.core.TagArray
+
+/**
+ * Kind 31890: Feed Definition Event (draft NIP, PR #1181).
+ *
+ * Addressable event that stores a custom feed definition.
+ * Content contains JSON-serialized feed source configuration.
+ * Tags include discoverability metadata (hashtags, authors, relays).
+ */
+@Immutable
+class FeedDefinitionEvent(
+    id: HexKey,
+    pubKey: HexKey,
+    createdAt: Long,
+    tags: TagArray,
+    content: String,
+    sig: HexKey,
+) : BaseAddressableEvent(id, pubKey, createdAt, KIND, tags, content, sig) {
+    fun title(): String? = tags.firstOrNull { it.size >= 2 && it[0] == "title" }?.get(1)
+
+    fun emoji(): String? = tags.firstOrNull { it.size >= 2 && it[0] == "emoji" }?.get(1)
+
+    fun feedConfigJson(): String = content
+
+    companion object {
+        const val KIND = 31890
+    }
+}