feat(chat): add encrypted invite contract

docs/CHAT.md

@@ -0,0 +1,72 @@
+# Ephemeral end-to-end encrypted chat
+
+KoalaSync chat is an optional, live-only text channel rendered on the selected
+streaming tab. It is not a messenger and does not add chat UI to the extension
+popup.
+
+## Security boundary
+
+- The relay receives ciphertext only and never receives the chat secret.
+- The relay stores no messages in RAM or on disk and sends no backlog.
+- A late joiner sees only messages sent after joining.
+- Room authentication is unchanged. The room password remains separate from the
+  chat secret and continues to be sent to the relay during `join_room`.
+- Message timing and ciphertext length remain visible to the relay. Same-room
+  replay is accepted by the threat model.
+
+## Invite format and key lifecycle
+
+New invitations use a named fragment format:
+
+```text
+#j2:r=<roomId>&p=<password>&k=<base64url-secret>[&u=<relayUrl>]
+```
+
+The room creator generates 16 random bytes and encodes them as 22 unpadded
+base64url characters. URL fragments are not sent in HTTP requests. The website
+parses the fragment and passes structured fields through `bridge.js` to the
+extension. `chatKey` must never be included in a relay event.
+
+Legacy `#join:` links remain supported and join normally without chat. Manual
+room/password entry also joins without chat. The extension clears any prior chat
+secret when joining without a key, switching rooms, or leaving.
+
+Deployment order is website first, extension second. An old extension ignores the
+additional structured `chatKey` field and continues joining normally.
+
+## Cryptography
+
+- Secret: 16 cryptographically random bytes.
+- KDF: HKDF-SHA256 with `roomId` as salt and a fixed KoalaSync chat info label.
+- Encryption: AES-256-GCM using WebCrypto.
+- IV: fresh random 12-byte value for every message, prepended to the ciphertext.
+- AAD: `${roomId}|${senderId}`.
+- The derived `CryptoKey` is cached once per active room.
+
+The relay stamps `id`, `senderId`, and `timestamp` on the ciphertext envelope.
+Changing `senderId` causes AES-GCM authentication to fail because the receiver uses
+the stamped value as AAD.
+
+## Client policy
+
+- Maximum plaintext length: 500 Unicode code points.
+- Decrypted text is untrusted. Escape HTML before applying the supported limited
+  Markdown formatting.
+- No read receipts and no typing indicators.
+- The local message DOM is bounded; this is presentation state, not server history.
+
+## Overlay behavior
+
+- Default dock: right.
+- Live modes: right, left, and detached.
+- Detached mode is draggable and moderately resizable. Size and position are stored
+  per origin and clamped to the current viewport.
+- The overlay uses a Shadow DOM and never changes host-page layout.
+- On `fullscreenchange`, its host moves into `document.fullscreenElement` and remains
+  visible.
+- It follows all eucalyptus, cyber, and graphite light/dark theme combinations.
+- Without a key, the panel stays closed and a disabled chat control explains that a
+  current invite link is required.
+- Without the relay `chat` capability, no chat control is shown.
+
+Peer removal and role management are outside chat scope.

docs/PROTOCOL.md

@@ -11,6 +11,19 @@ the event names defined in `shared/constants.js`.
   object payload.
 - The relay caps incoming Socket.IO message size at 4 KB.
 
+## Invite fragments
+
+Current invitations use named URL-fragment fields:
+
+```text
+#j2:r=<roomId>&p=<password>&k=<base64url-chat-secret>[&u=<relayUrl>]
+```
+
+The fragment is parsed by the website and forwarded to the extension as structured
+fields. `k` is client-only and must never appear in a relay payload. Legacy
+`#join:<roomId>:<password>[:1:<relayUrl>]` fragments remain valid but provide no chat
+secret.
+
 ## Connection Handshake
 
 The Socket.IO handshake must include: