feat: Add support for message attachments

- Introduced Attachment model to handle file attachments associated with messages.
- Updated ModerationController to manage message deletions and attachment cleanup.
- Enhanced ChatService to include attachments in message retrieval.
- Implemented migration for legacy single-attachment messages to the new Attachments model.
- Added unit tests for attachment handling in message formatting and parsing.
- Updated database context and migrations to support new Attachments table.

docs/changelog/v0.2.12.md

@@ -13,12 +13,18 @@ Private channels are now genuinely private: password-protected channels are end-
   - End-to-end encrypted channels cannot be joined over the IRC gateway (that would require the server to hold the room key) — IRC `JOIN` returns `475` directing users to the EchoHub client.
 - Password-protected channels — set an optional password when creating a channel (masked field in the Create Channel dialog, `password` on `POST /api/channels`). Passwords are BCrypt-hashed server-side; the join gate applies on first join only (existing members and the creator are unaffected). Protected channels show a `*` marker in the channel list and `+k` in the status bar
 - Save original images — image messages now show a clickable "[↓ save original]" line under the ASCII-art preview that downloads the full-resolution original to your Downloads folder (decrypting locally in encrypted channels)
+- **Messages with attachments (Discord-style)** — a message is now text **plus** a list of attachments instead of being either text or a single file. One message can carry a caption and several files (images, audio, docs) together:
+  - Compose with a **staging tray**: `/send <file>` or dropping files onto the terminal stages them (shown on the input bar); the next Enter sends your typed caption and all staged files as one message. `/clear` drops staged files. `/send <URL>` still posts an image immediately.
+  - Each image attachment renders its own ASCII preview with its own "save original" action; audio/file attachments each get their own play/download line.
+  - In encrypted channels every attachment is encrypted individually (blob + ASCII preview), and the caption is room-encrypted — the server still stores only ciphertext and can report count/size but not contents.
+  - Up to 10 attachments per message.
+- **Message deletion** — press Delete on a selected message to remove it. You can always delete your own messages; moderators and above can delete others' messages, but only from users **below their own role** (a mod can't delete an admin's or owner's message). Deleting a message also removes its attachment blobs from server storage.
+- **Customizable download folder** — `/downloadpath` opens your OS-native folder picker (Windows Explorer / macOS Finder / Linux GTK or KDE) to choose where downloaded attachments and saved images go; `/downloadpath <path>` sets it directly (the fallback when no native picker is available). Downloaded files now land in that folder (with automatic `(n)` de-duplication) instead of a temp directory.
 - `/join <channel> [password]` — join protected channels inline, or let the client prompt: joining a protected channel without a password opens a masked prompt that re-prompts on a wrong password
 - IRC channel keys — `JOIN #room <key>` works against room passwords (RFC 1459 comma-paired key lists supported); keyless or wrong-key joins get `475 ERR_BADCHANNELKEY`
 - IRC `MODE` implemented — `MODE #chan` reports `+k`/`+`, `MODE #chan +k <key>` sets and `-k` clears the room password (channel creator or admin only), ban-list probes get a clean empty reply, and `CHANMODES` is advertised in ISUPPORT
 - IRC `TOPIC` set support — the channel creator can change the topic from IRC; the change broadcasts to connected TUI clients (previously topic changes were rejected with a stub error)
-- Drag & drop file sending — dropping a file (image, audio, anything) onto the terminal detects the pasted path and sends it through `/send` automatically, including multiple files at once
-- Ctrl+V pastes into the message input (previously paste was only available via the right-click menu); Ctrl+Y works as an alias
+- Attach a file by drag & drop or by pasting — drop a file onto the terminal, or **copy a file in your file manager and press Ctrl+V**, to stage it as an attachment (the next Enter sends it with your caption). Multiple files at once are supported. Ctrl+V still pastes text when the clipboard holds text; Ctrl+Y is a paste alias. On Windows the copied-file paste reads the clipboard's file list directly (Windows Terminal never pastes copied files as text), with `xclip`/`wl-paste` used on Linux
 - New `TransparentLight` theme — dark characters on a transparent background, for light terminal color schemes (`/theme transparentlight`)
 - Timestamps in messages are now aware of the current culture and display the short time pattern for today's messages and the short date+time pattern for older messages.
 
@@ -34,4 +40,6 @@ Private channels are now genuinely private: password-protected channels are end-
 - New endpoints: `GET /api/channels/{channel}/crypto` (public crypto metadata — salt only, never the wrapped key) and `POST /api/channels/{channel}/rekey` (creator-only passphrase change)
 - The upload endpoint accepts `type` and `content` form fields for encrypted channels, where the client supplies the declared message type and room-encrypted content
 - `ImageToAsciiService` and `FileValidationHelper` moved from `EchoHub.Server` to `EchoHub.Core` so the client can render ASCII art and detect file types for encrypted uploads
-- New EF migrations `AddChannelPasswordHash` and `AddChannelEncryptionEnvelope` (applied automatically on server start)
+- **Message shape change**: `MessageDto` drops `Type`/`AttachmentUrl`/`AttachmentFileName`/`AttachmentFileSize` and gains `Attachments` (a list of `AttachmentDto { Kind, Url, FileName, FileSize, AsciiPreview }`, null/empty for plain text). New `Attachment` entity + table with a cascade FK to `Message`
+- New endpoint `POST /api/channels/{channel}/messages` (multipart: `content` + N `files`, plus `kind`/`preview` per file for encrypted channels) replaces the single-file `upload` endpoint; `DELETE /api/moderation/messages/{id}` now enforces the own-or-higher-role rule
+- New EF migrations `AddChannelPasswordHash`, `AddChannelEncryptionEnvelope`, and `AddMessageAttachments` (applied automatically on server start); a one-time startup data migration folds legacy single-attachment messages into the new model