feat: Add notification sound functionality and embed support

- Introduced NotificationConfig to manage notification settings, including sound file.
- Added Notification.mp3 asset for notifications.
- Implemented NotificationSoundService to handle playing notification sounds.
- Enhanced ChatRenderer and MainWindow to support emoji rendering.
- Updated MessageDto to allow multiple embeds per message.
- Modified LinkEmbedService to fetch multiple embeds from message content.
- Added data migration for legacy embed JSON format to new array format.
- Adjusted embed handling in ChatService and IrcMessageFormatter to accommodate multiple embeds.
- Updated HubConstants for new embed dimensions and limits.

docs/changelog/v0.2.4.md

@@ -0,0 +1,22 @@
+# v0.2.4 - Link Embeds
+
+## Features
+
+### OpenGraph Link Embeds
+- Messages containing URLs now show a rich preview below the message text, similar to Discord
+- Server-side fetching: detects the first URL in a message, fetches the page, and parses OpenGraph meta tags (`og:title`, `og:description`, `og:image`, `og:site_name`)
+- OG images are converted to a small 24x12 colored ASCII thumbnail using the existing half-block renderer
+- Embeds are persisted in the database and included in channel history
+- TUI client renders embeds with a `▏` left border bar — site name and border in blue, title in white, description in gray, followed by the ASCII thumbnail
+- IRC gateway receives a text-only embed preview (site name, title, description) — no ASCII thumbnail to keep IRC output clean
+- Falls back to `<title>` tag when no OG tags are present; gracefully skips if no useful metadata is found
+- 3-second fetch timeout ensures message delivery is never significantly delayed
+- SSRF protection rejects private/loopback IP addresses before fetching
+
+## Infrastructure
+
+- New `LinkEmbedService` on the server — URL detection, HTML fetching (first 64KB), OG tag parsing via regex, image thumbnail generation
+- `EmbedDto` record added to shared Core DTOs; `MessageDto` extended with optional `Embed` field (backward-compatible)
+- `EmbedJson` nullable column on the `Message` table stores serialized embed data (max 8KB)
+- Dedicated `"OgFetch"` named HttpClient with bot User-Agent header and 5-second timeout
+- New EF Core migration: `AddMessageEmbed`