fix(calls): remove misleading Retry button from call load error overlay (N96)

Both Retry and Leave called the same dismiss function; Retry implied a
reconnect attempt that never happened. Collapsed to a single Back button
that honestly describes returning to the prescreen.

docs: correct Gemini audit entries — sanitize-html not DOMPurify (Claim A),
retract inaccurate LiveKit replaceTrack soundboard approach (Claim B,
contradicts confirmed cross-origin iframe constraint), expand N95 fix note
to clarify track-stop vs AudioContext-suspend distinction.

docs(testing): add L1 N95 reproduction guide; update A7 to reflect single Back button.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/features/call/CallView.tsx

@@ -164,7 +164,7 @@ function CallLoadErrorMessage() {
   const setCallEmbed = useSetAtom(callEmbedAtom);
 
   // Disposing the embed tears down the hung iframe and returns the user to the
-  // prescreen, from which they can join again ("Retry") or simply walk away.
+  // prescreen, where they can choose to join again.
   const dismiss = () => setCallEmbed(undefined);
 
   return (
@@ -180,11 +180,8 @@ function CallLoadErrorMessage() {
         The call failed to load. Check your connection and try again.
       </Text>
       <Box gap="200" alignItems="Center">
-        <Button variant="Primary" size="300" radii="400" onClick={dismiss}>
-          <Text size="B400">Retry</Text>
-        </Button>
         <Button variant="Secondary" fill="Soft" size="300" radii="400" onClick={dismiss}>
-          <Text size="B400">Leave</Text>
+          <Text size="B400">Back</Text>
         </Button>
       </Box>
     </Box>