feat: route [text](#hashtag) markdown links to the hashtag feed (#1564)

Markdown links whose target is a bare hashtag fragment, e.g.
[Nostr Multiplayer Games](#NostrMultiplayerGames), did nothing when
tapped: uriToRoute had no branch for them, so the tap fell through to
the system uri handler, which silently fails on a bare fragment.

- uriToRoute now resolves a bare #tag target to Route.Hashtag, reusing
  RichTextParser.hashTagsPattern so tag validity matches how #hashtags
  linkify in plain text (including unicode tags). Full URLs with
  anchors are unaffected.
- MarkdownMediaRenderer rewrites the link destination to
  nostr:hashtag?id=<tag> (mirroring renderHashtag) while keeping the
  custom link text, so the tap flows through the same route string as
  a plain #hashtag.

Read side only: the composer still never emits this syntax.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01D382kMNdcmMLZrwKeZAPTY

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/MainActivity.kt

@@ -27,6 +27,7 @@ import androidx.activity.enableEdgeToEdge
 import androidx.annotation.RequiresApi
 import androidx.appcompat.app.AppCompatActivity
 import com.vitorpamplona.amethyst.Amethyst
+import com.vitorpamplona.amethyst.commons.richtext.RichTextParser
 import com.vitorpamplona.amethyst.debugState
 import com.vitorpamplona.amethyst.model.Account
 import com.vitorpamplona.amethyst.model.LocalCache
@@ -153,6 +154,18 @@ fun isNotificationRoute(uri: String) = uri.startsWith("notifications", true) ||
 
 fun isHashtagRoute(uri: String) = uri.startsWith("hashtag?id=") || uri.startsWith("nostr:hashtag?id=")
 
+/**
+ * A markdown link target that is a bare hashtag fragment, e.g. the `#NostrMultiplayerGames` in
+ * `[Games](#NostrMultiplayerGames)`. Returns the tag without the leading `#`, or null when the
+ * uri is anything else (full URLs with anchors don't start with `#`). Reuses the same character
+ * class that linkifies #hashtags in plain text, so validity matches how tags parse everywhere else.
+ */
+fun fragmentHashtagOrNull(uri: String): String? {
+    val match = RichTextParser.hashTagsPattern.matchEntire(uri) ?: return null
+    if (!match.groups[2]?.value.isNullOrEmpty()) return null
+    return match.groups[1]?.value
+}
+
 fun isUrlRoute(uri: String) = uri.startsWith("url?id=") || uri.startsWith("nostr:url?id=")
 
 fun isConnectedAppRoute(uri: String) = uri.startsWith("connectedapp?coordinate=") || uri.startsWith("nostr:connectedapp?coordinate=")
@@ -199,6 +212,9 @@ fun uriToRoute(
     if (isHashtagRoute(uri)) {
         return Route.Hashtag(uri.removePrefix(NOSTR_URI_PREFIX).removePrefix("hashtag?id=").lowercase())
     }
+    fragmentHashtagOrNull(uri)?.let {
+        return Route.Hashtag(it.lowercase())
+    }
     if (isUrlRoute(uri)) {
         return urlRoute(uri)
     }

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/components/markdown/MarkdownMediaRenderer.kt

@@ -43,6 +43,7 @@ import com.vitorpamplona.amethyst.ui.components.DisplayFullNote
 import com.vitorpamplona.amethyst.ui.components.DisplayUser
 import com.vitorpamplona.amethyst.ui.components.LoadUrlPreview
 import com.vitorpamplona.amethyst.ui.components.ZoomableContentView
+import com.vitorpamplona.amethyst.ui.fragmentHashtagOrNull
 import com.vitorpamplona.amethyst.ui.navigation.navs.INav
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.AccountViewModel
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.LoadedBechLink
@@ -74,7 +75,10 @@ class MarkdownMediaRenderer(
         title: String?,
         uri: String,
     ): Boolean =
-        if (canPreview && uri.startsWith("http")) {
+        if (fragmentHashtagOrNull(uri) != null) {
+            // claims [text](#hashtag) links so renderLinkPreview can retarget them to the hashtag feed
+            true
+        } else if (canPreview && uri.startsWith("http")) {
             title.isNullOrBlank() || title == uri
         } else {
             false
@@ -115,6 +119,12 @@ class MarkdownMediaRenderer(
         uri: String,
         richTextStringBuilder: RichTextString.Builder,
     ) {
+        val fragmentHashtag = fragmentHashtagOrNull(uri)
+        if (fragmentHashtag != null) {
+            renderAsCompleteLink(title ?: uri, "nostr:hashtag?id=$fragmentHashtag", richTextStringBuilder)
+            return
+        }
+
         val content = parser.createMediaContent(uri, imetaByUrl, startOfText, callbackUri)
 
         if (canPreview) {

amethyst/src/test/java/com/vitorpamplona/amethyst/ui/UriToRouteTest.kt

@@ -0,0 +1,70 @@
+/*
+ * 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.ui
+
+import com.vitorpamplona.amethyst.model.Account
+import com.vitorpamplona.amethyst.ui.navigation.routes.Route
+import io.mockk.mockk
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
+import org.junit.Test
+
+class UriToRouteTest {
+    private val account = mockk<Account>()
+
+    @Test
+    fun fragmentHashtagRoutesToHashtagFeed() {
+        assertEquals(Route.Hashtag("nostrmultiplayergames"), uriToRoute("#NostrMultiplayerGames", account))
+    }
+
+    @Test
+    fun fragmentHashtagKeepsUnicodeTags() {
+        assertEquals(Route.Hashtag("日本語"), uriToRoute("#日本語", account))
+    }
+
+    @Test
+    fun invalidFragmentHashtagsAreNotRoutes() {
+        assertNull(uriToRoute("#", account))
+        assertNull(uriToRoute("# ", account))
+        assertNull(uriToRoute("#two words", account))
+    }
+
+    @Test
+    fun fullUrlsWithAnchorsAreNotHashtagRoutes() {
+        assertNull(uriToRoute("https://example.com/page#anchor", account))
+    }
+
+    @Test
+    fun hashtagQueryRoutesStillWork() {
+        assertEquals(Route.Hashtag("foo"), uriToRoute("hashtag?id=foo", account))
+        assertEquals(Route.Hashtag("foo"), uriToRoute("nostr:hashtag?id=foo", account))
+    }
+
+    @Test
+    fun fragmentHashtagOrNullExtractsTheTag() {
+        assertEquals("NostrMultiplayerGames", fragmentHashtagOrNull("#NostrMultiplayerGames"))
+        assertNull(fragmentHashtagOrNull("#"))
+        assertNull(fragmentHashtagOrNull("##double"))
+        assertNull(fragmentHashtagOrNull("#tag!"))
+        assertNull(fragmentHashtagOrNull("https://example.com/page#anchor"))
+        assertNull(fragmentHashtagOrNull("nostr:hashtag?id=foo"))
+    }
+}