Move Windows camera help into overlay

The privacy instructions no longer bloat the launcher card.
A "Help" button in the navbar opens them in a native dialog
(Esc, backdrop and button close it). The button only shows
for Windows users, reusing the existing OS detection.

Translations may contain <kbd>, <b> and <i> markup now. The
renderer builds nodes from a tag whitelist instead of using
innerHTML, so smuggled tags from i18n.json cannot execute.

All help texts live under meetingPrivacySettings.windows;
sibling keys for other operating systems can follow.

assets/css/launch.css

@@ -206,13 +206,26 @@ button, .button {
   background: #f5f5f5;
 }
 
-/* Long text needs left alignment, the card is centered */
-.privacy {
-  margin-top: 24px;
-  padding: 20px;
-  border-radius: 8px;
+/* Windows help overlay – long text needs left
+ * alignment, the page is centered */
+dialog.privacy {
+  width: min(1000px, calc(100% - 32px));
+  max-height: calc(100vh - 48px);
+  border: 0;
+  border-radius: 14px;
   background: #f5f5f5;
+  padding: 20px;
   text-align: left;
+  overflow-y: auto;
+}
+
+dialog.privacy::backdrop {
+  background: rgba(0, 0, 0, 0.5);
+}
+
+.help-actions {
+  margin-top: 24px;
+  text-align: center;
 }
 
 .room {
@@ -222,6 +235,17 @@ button, .button {
   word-break: break-all;
 }
 
+/* Keyboard keys, marked with <kbd> in i18n.json */
+kbd {
+  padding: 1px 6px;
+  border: 1px solid #ccc;
+  border-bottom-width: 3px;
+  border-radius: 4px;
+  background: #f0f0f0;
+  font-family: monospace;
+  font-size: 0.9em;
+}
+
 .error {
   padding: 16px;
   border-radius: 8px;
@@ -470,6 +494,11 @@ button, .button {
   color: #bbb;
 }
 
+:root.dark kbd {
+  border-color: #555;
+  background: #404040;
+}
+
 :root.dark .secondary {
   background: #444;
   color: #eee;