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

locales/zh.json

@@ -31,6 +31,9 @@
   },
   "ui": {
     "language.label": "语言",
+    "language.suggest.text": "本页面也有简体中文版本。",
+    "language.suggest.cta": "阅读简体中文",
+    "language.suggest.dismiss": "关闭",
     "community.title": "一起构筑隐私的未来",
     "community.description": "SecureBit 因社区而成长。你的想法与反馈塑造着安全通讯的未来 —— 开放开发，全程完整的 ASN.1 校验。",
     "community.github": "GitHub 仓库",
@@ -776,7 +779,29 @@
     "offline.disconnect": "断开连接",
     "offline.learnMore": "了解更多",
     "pwa.installApp": "安装应用",
-    "chat.onWeb": "你正在使用网页版"
+    "chat.onWeb": "你正在使用网页版",
+    "groupCall.startVoice": "发起群组语音通话",
+    "groupCall.startVideo": "发起群组视频通话",
+    "groupCall.join": "加入",
+    "groupCall.dismiss": "暂不加入",
+    "groupCall.leave": "离开通话",
+    "groupCall.you": "你",
+    "groupCall.connecting": "连接中…",
+    "groupCall.waitingLink": "正在等待直连…",
+    "groupCall.legFailed": "无法连接",
+    "groupCall.startedVoice": "{name} 发起了语音通话",
+    "groupCall.startedVideo": "{name} 发起了视频通话",
+    "groupCall.inCall": "{count} 人在通话中",
+    "groupCall.err.permission_denied": "通话无法开始 - 麦克风和摄像头权限被拒绝。请为本站点允许后重试。",
+    "groupCall.err.device_not_found": "通话无法开始 - 此设备上未找到麦克风。",
+    "groupCall.err.device_busy": "通话无法开始 - 麦克风正被其他应用占用。请关闭后重试。",
+    "groupCall.err.media_failed": "通话无法开始 - 无法打开麦克风。",
+    "groupCall.err.call_in_progress": "该群组已有通话正在进行。请加入，而不是另开一个。",
+    "groupCall.err.not_ready": "请先确认群组安全码再通话。",
+    "groupCall.speaking": "{name} 正在讲话",
+    "groupCall.pin": "放大显示 {name}",
+    "groupCall.unpin": "返回所有人",
+    "groupCall.showEveryone": "所有人"
   },
   "manifest": {
     "name": "SecureBit.chat - 私密的端到端加密通讯工具",