feat(groups): audio and video calls in group chats; release v6.6.6

A group call is N-1 ordinary 1:1 calls, one to each other member, each riding
the pairwise session that member already has — a transport a human already
authenticated by comparing the safety code. No mixer, no SFU, no point at which
two people's media meets anywhere but on a device.

Call control is separate from call media, because the two reach different sets
of people. Who opened a call, who joined and who left travels as group frames
signed with the sender's group identity key, so it reaches members currently
reachable only through a relay — and a relaying member can drop one but cannot
write one. Media flows only where a direct link exists, so a member without one
shows as connecting rather than being omitted. Frames carry a per-sender
sequence checked before the action, so a captured leave cannot end a later call,
and simultaneous calls converge on the lower random call id.

One capture is shared across every leg rather than one getUserMedia per member,
and legs answer without prompting: the flag permitting that is set only locally,
only while this user is in the call, and cleared when they leave.

UI: a gallery that sizes itself from the space it has, a spotlight view, an
active-speaker indicator read from the waveform, and the call surface in the
same visual language as the 1:1 one.

Also in this commit, the v6.5.0 language-suggestion work that had not been
pushed yet; its notes are in the changelog. And two fixes: the safety-code input
asks for digits rather than text, and starting a new chat from inside a group no
longer creates it behind the group where it cannot be seen — which had made it
impossible to connect to anyone new, or to add anyone to a group, while a group
was open.

Claude-Session: https://claude.ai/code/session_01XSxAkET3hQTkYDQfbjCQwZ

README.md

@@ -9,7 +9,7 @@
 No accounts. No servers storing your messages. No installation required.
 
 [![License: MIT](https://img.shields.io/badge/License-MIT-f0892a.svg)](LICENSE)
-[![Version](https://img.shields.io/badge/version-6.4.0-3ecf8e.svg)](CHANGELOG.md)
+[![Version](https://img.shields.io/badge/version-6.6.6-3ecf8e.svg)](CHANGELOG.md)
 [![PWA](https://img.shields.io/badge/PWA-installable-3ecf8e.svg)](#install-as-an-app)
 [![Encryption](https://img.shields.io/badge/crypto-ECDH%20P--384%20%C2%B7%20AES--256--GCM-blue.svg)](#security-model)
 [![Forward secrecy](https://img.shields.io/badge/forward%20secrecy-Double%20Ratchet-3ecf8e.svg)](#forward-secrecy)
@@ -50,6 +50,8 @@ It is designed for people who need a small, auditable, zero-infrastructure way t
 - **Adaptive audio**: Opus with in-band FEC, DTX and RED redundancy for intelligible speech at 15 to 20 percent packet loss; audio is prioritised and never throttled by the network controller.
 - **Adaptive video**: VP9/AV1 single-encoding SVC (H.264/VP8 fallback) that degrades by spatial/temporal layer, with a runtime controller that trims video bitrate on loss/RTT and recovers as the link clears.
 - **Live connection-quality indicator** (Excellent, Good, Fair, Weak) shown during a call, plus in-call mute and video controls.
+- **Group voice and video calls**, built as one encrypted call per member rather than a conference server: media flows directly between each pair over the link they already verified, so no machine in the middle ever holds the room. Who opened a call and who is in it is signed with each member's group key, so a member relaying that message for two people who cannot reach each other can drop it but never forge it.
+- **One microphone capture for the whole group call**, shared across every leg, released the moment you leave — you are asked for the microphone once, not once per participant.
 
 **Messaging**
 - **Encrypted voice messages.** Record in the browser and send over the same end-to-end encrypted transfer channel as files. Audio is captured as PCM/WAV, integrity-protected by a signed hash, and played back inline on the recipient's device without ever touching disk.