Render custom emoji inline and play animated images

[malkoG]

Closes #166

Summary

Custom emoji rendering (HtmlContent.kt)

• hackers.pub serves custom emoji as <img alt=":name:" style="height:1em;vertical-align:middle;display:inline-block;"> (renderCustomEmojis in models/emoji.ts). There is no class, because sanitization strips class/style from <img> before emoji are injected.
• The app only treated <img> as inline when it sat inside a link/mention or had class="inline-block". As a result:
  • Feed / previews (Bug 3): emoji were silently dropped.
  • Post detail (Bug 1): emoji were pulled out as full-width image blocks, which split the paragraph and made them huge.
• Emoji are now recognized by an alt shortcode (or the web frontend's class="emoji") and kept in the text flow. They are sized relative to the surrounding text (1.2em) instead of a fixed 18dp circle crop. Mention avatars are unchanged.

Animated images (Bug 2)

• Adds coil-gif, which registers itself through Coil's service loader:
  • API 28+: AnimatedImageDecoder (GIF, animated WebP, and animated HEIF on API 30+)
  • API 26–27: GifDecoder (GIF only)
• Applies app-wide, so animated custom emoji, remote avatars, and inline images now play.

Notes

• Local hackers.pub avatars stay static. The server's transformAvatar reads only the first frame, and AvatarPicker re-encodes uploads as JPEG. That is out of scope here.
• Reaction notifications with a custom emoji still show only the text. NotificationsScreen uses Notification.React.emoji and ignores customEmoji. Left for a follow-up.

Test plan

• HtmlContentKtTest: new cases for server emoji markup, class="emoji", and plain body images still leaving the text flow
• ./gradlew :app:lintDebug
• Full unit test suite (verified locally)
• Emulator (API 33): an animated GIF remote avatar plays on the profile screen
• Device: confirmed working
• Release (minified) build keeps the coil-gif service loader registration

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📥 Commits

Reviewing files that changed from the base of the PR and between f1f76bc and bbbbcb2.

📒 Files selected for processing (4)

• app/build.gradle.kts
• app/src/main/java/pub/hackers/android/ui/components/HtmlContent.kt
• app/src/test/java/pub/hackers/android/ui/components/HtmlContentKtTest.kt
• gradle/libs.versions.toml

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