feat: Filter home feed by hashtag in-place instead of navigating away

Same approach as relay filtering: selecting a hashtag from the top nav
filter now filters the home feed in-place across all tabs, with
subscriptions scoped to that hashtag.

https://claude.ai/code/session_01PZLLjE7MWh7PPz1woVqmxM

amethyst/src/main/java/com/vitorpamplona/amethyst/model/topNavFeeds/FeedTopNavFilterState.kt

@@ -30,6 +30,7 @@ import com.vitorpamplona.amethyst.model.topNavFeeds.allUserFollows.Kind3UserFoll
 import com.vitorpamplona.amethyst.model.topNavFeeds.aroundMe.AroundMeFeedFlow
 import com.vitorpamplona.amethyst.model.topNavFeeds.chess.ChessFeedFlow
 import com.vitorpamplona.amethyst.model.topNavFeeds.global.GlobalFeedFlow
+import com.vitorpamplona.amethyst.model.topNavFeeds.hashtag.HashtagFeedFlow
 import com.vitorpamplona.amethyst.model.topNavFeeds.noteBased.NoteFeedFlow
 import com.vitorpamplona.amethyst.model.topNavFeeds.relay.RelayFeedFlow
 import com.vitorpamplona.amethyst.model.topNavFeeds.unknown.UnknownFeedFlow
@@ -135,7 +136,7 @@ class FeedTopNavFilterState(
             }
 
             is TopFilter.Hashtag -> {
-                UnknownFeedFlow(listName)
+                HashtagFeedFlow(listName.tag, followsRelays, proxyRelays)
             }
 
             is TopFilter.Relay -> {

amethyst/src/main/java/com/vitorpamplona/amethyst/model/topNavFeeds/hashtag/HashtagFeedFlow.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.amethyst.model.topNavFeeds.hashtag
+
+import com.vitorpamplona.amethyst.model.topNavFeeds.IFeedFlowsType
+import com.vitorpamplona.amethyst.model.topNavFeeds.IFeedTopNavFilter
+import com.vitorpamplona.quartz.nip01Core.relay.normalizer.NormalizedRelayUrl
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.FlowCollector
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.combine
+
+class HashtagFeedFlow(
+    val hashtag: String,
+    val outboxRelays: StateFlow<Set<NormalizedRelayUrl>>,
+    val proxyRelays: StateFlow<Set<NormalizedRelayUrl>>,
+) : IFeedFlowsType {
+    fun buildFilter(relays: Set<NormalizedRelayUrl>) = HashtagTopNavFilter(setOf(hashtag), relays)
+
+    override fun flow(): Flow<IFeedTopNavFilter> =
+        combine(outboxRelays, proxyRelays) { outbox, proxy ->
+            if (proxy.isNotEmpty()) buildFilter(proxy) else buildFilter(outbox)
+        }
+
+    override fun startValue(): HashtagTopNavFilter =
+        if (proxyRelays.value.isNotEmpty()) {
+            buildFilter(proxyRelays.value)
+        } else {
+            buildFilter(outboxRelays.value)
+        }
+
+    override suspend fun startValue(collector: FlowCollector<IFeedTopNavFilter>) {
+        collector.emit(startValue())
+    }
+}

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/TopNavFilterState.kt

@@ -146,7 +146,6 @@ class TopNavFilterState(
                 FeedDefinition(
                     TopFilter.Hashtag(it),
                     HashtagName(it),
-                    route = Route.Hashtag(it),
                 )
             }