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

tests/mobile-chat-layout.test.mjs

@@ -155,12 +155,20 @@ assert.match(app, /webrtcManager: null/,
 // the connection toast must not sit on the header
 //
 // It is `fixed top-4 left-1/2` (src/pwa/pwa-manager.js), which inside the chat
-// lands on the 64px header and covers the peer name.
+// lands on the header and covers the peer name. The offset is expressed in the
+// same two variables the header's own height is (--sb-bar-h for the content row,
+// --sb-safe-top for the status-bar strip an installed app draws under), so the
+// toast follows the bar instead of restating a number that only holds in a
+// browser tab.
 // ---------------------------------------------------------------------------
 {
     const css = readFileSync(new URL('../src/styles/components.css', import.meta.url), 'utf8');
-    assert.match(css, /body\.sb-in-chat #pwa-connection-status\s*\{[^}]*top:\s*calc\(64px/,
-        'the online/offline toast must clear the chat header');
+    const rule = css.match(/body\.sb-in-chat #pwa-connection-status\s*\{[^}]*\}/);
+    assert.ok(rule, 'the online/offline toast must be offset inside the chat');
+    assert.match(rule[0], /top:\s*calc\([^)]*var\(--sb-bar-h/,
+        'the toast offset must track the header height, not a hard-coded 64px');
+    assert.match(rule[0], /var\(--sb-safe-top/,
+        'the toast must also clear the status-bar strip in an installed app');
 }
 
 // ---------------------------------------------------------------------------