mastodon-web-like trailing hashtag bar (#4761)

Rationale: Since the mastodon web UI has started stripping "trailing"
hashtags from post content and shoving it into an ellipsized section at
the bottom of posts, the general hashtag : content ratio is rising.

This is an attempt at adopting a similar functionality for Tusky.

Before:

<img width="420" alt="Screenshot of a hashtag-heavy post on Tusky
nightly"
src="https://github.com/user-attachments/assets/09c286e8-6822-482a-904c-5cb3323ea0e1">


After:
![Screenshot of the same post on this
branch](https://github.com/user-attachments/assets/fa99964d-a057-4727-b9f0-1251a199d5f8)

app/src/test/java/com/keylesspalace/tusky/util/LinkHelperTest.kt

@@ -315,6 +315,74 @@ class LinkHelperTest {
         }
     }
 
+    @Test
+    fun `get trailing hashtags with empty content returns empty list`() {
+        assert(getTrailingHashtags(SpannableStringBuilder("")).second.isEmpty())
+    }
+
+    @Test
+    fun `get trailing hashtags with no hashtags returns empty list`() {
+        assert(getTrailingHashtags(SpannableStringBuilder("some untagged content")).second.isEmpty())
+    }
+
+    @Test
+    fun `get trailing hashtags with all inline hashtags returns empty list`() {
+        assert(getTrailingHashtags(SpannableStringBuilder("some #inline #tagged #content")).second.isEmpty())
+    }
+
+    @Test
+    fun `get trailing hashtags with one tag`() {
+        val content = SpannableStringBuilder("some content followed by tags:\n").apply {
+            tags.first().let { append("#${it.name}", URLSpan(it.url), 0) }
+        }
+
+        val (_, trailingHashtags) = getTrailingHashtags(content)
+        assertEquals(tags.first().name, trailingHashtags.single().name)
+        assertEquals(tags.first().url, trailingHashtags.single().url)
+    }
+
+    @Test
+    fun `get trailing hashtags with multiple tags`() {
+        for (separator in listOf(" ", "\t", "\n", "\r\n")) {
+            val content = SpannableStringBuilder("some content followed by tags:\n").apply {
+                for (tag in tags) {
+                    append(separator)
+                    append("#${tag.name}", URLSpan(tag.url), 0)
+                    append(separator)
+                }
+            }
+
+            val (_, trailingHashtags) = getTrailingHashtags(content)
+            assertEquals(tags.size, trailingHashtags.size)
+            tags.forEachIndexed { index, tag ->
+                assertEquals(tag.name, trailingHashtags[index].name)
+                assertEquals(tag.url, trailingHashtags[index].url)
+            }
+        }
+    }
+
+    @Test
+    fun `get trailing hashtags ignores inline tags`() {
+        for (separator in listOf(" ", "\t", "\n", "\r\n")) {
+            val content = SpannableStringBuilder("some content with inline tag ").apply {
+                append("#inline", URLSpan("https://example.com/tag/inline"), 0)
+                append(" followed by trailing tags\n")
+                for (tag in tags) {
+                    append(separator)
+                    append("#${tag.name}", URLSpan(tag.url), 0)
+                    append(separator)
+                }
+            }
+
+            val (_, trailingHashtags) = getTrailingHashtags(content)
+            assertEquals(tags.size, trailingHashtags.size)
+            tags.forEachIndexed { index, tag ->
+                assertEquals(tag.name, trailingHashtags[index].name)
+                assertEquals(tag.url, trailingHashtags[index].url)
+            }
+        }
+    }
+
     @RunWith(Parameterized::class)
     class UrlMatchingTests(private val url: String, private val expectedResult: Boolean) {
         companion object {