feat: Implement end-to-end encryption for channels

- Added EncryptionSalt and WrappedRoomKey properties to Channel model.
- Introduced RoomCrypto class for client-side encryption and decryption.
- Updated ChannelService to handle encrypted channels, including creation and rekeying.
- Modified ChannelsController to expose crypto metadata and rekey functionality.
- Enhanced IrcCommandHandler to block joining encrypted channels over IRC.
- Updated database schema with migration for new encryption fields.
- Refactored file validation and image processing services to accommodate encrypted channels.
- Added unit tests for RoomCrypto functionality and updated existing tests for channel services.

docs/changelog/v0.2.12.md

@@ -1,5 +1,37 @@
 # v0.2.12
 
+Private channels are now genuinely private: password-protected channels are end-to-end encrypted, so the server (and its operators) can gate joins and measure storage but cannot read message or file contents. The IRC gateway grows real MODE/TOPIC support and channel keys, and the client gets image "save original", a transparent-light theme, drag-and-drop file sending, Ctrl+V paste, and a fix for the intermittent Ctrl+W crash.
+
 ## New Features
 
+- **End-to-end encrypted channels** — creating a channel with a password now provisions a zero-knowledge room:
+  - The passphrase never leaves the client. It derives (PBKDF2-SHA256, 210k iterations) two keys: an *auth key* sent to the server as the join credential, and a *key-encryption key* that never leaves the machine.
+  - A random room content key encrypts every message and file with AES-256-GCM. The server only ever stores the room key *wrapped* under the passphrase, so it can gate joins and report a channel's message count, storage size, and attachments — but cannot decrypt any of it. Even the server owner cannot read a private room's contents.
+  - Members' clients cache the derived room key locally (in the per-server config, like saved sessions) so the passphrase isn't retyped every launch; joining on a new device prompts for it once.
+  - Change the passphrase with `/passwd <old> <new>` (channel creator only). The room key is re-wrapped, not rotated, so **existing history stays readable** and members who join later with the new passphrase can still read older messages.
+  - Files and images are encrypted client-side before upload; for images the ASCII-art preview is rendered on the client and stored room-encrypted too. Sending images by URL is disabled in encrypted channels (the server can't fetch-and-render without the key).
+  - 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)
+- `/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
+- 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.
+
+## Bug Fixes
+
+- Fixed intermittent crash on Ctrl+W — Terminal.Gui binds Ctrl+W to clipboard-cut, and Windows clipboard contention (another app holding the clipboard) threw an unhandled `Win32Exception` that took the app down. Ctrl+W now deletes the previous word (readline behavior, no clipboard), and all clipboard shortcuts (Ctrl+X/C/V/Y) are guarded so transient clipboard failures log a warning instead of crashing
+- Fixed emoji shortcode replacement permanently disabling itself if a cursor update threw mid-replacement
+- IRC `LIST` no longer leaks private channels; protected channels are marked `[+k]`
+
+## API Changes
+
+- `ChannelDto` gains `isProtected` and `isEncrypted`; `CreateChannelRequest` gains optional `password`, `encryptionSalt`, and `wrappedRoomKey`; SignalR `JoinChannel` takes an optional second `password` argument and `JoinChannelResult` gains `passwordRequired`, `encryptionSalt`, and `wrappedRoomKey` (older clients must update to join over SignalR)
+- 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)