fix(a11y): announce the incoming-call overlay (role=alertdialog + name)

The full-screen incoming-call overlay (shown when you're not already in a
call — the common case) had no role or accessible name on any ancestor of
its buttons, so a screen reader landed on an unlabelled button with no
hint that a call was ringing. The in-call banner variant already had
role="alert" + a label. The overlay's Dialog is now role="alertdialog",
aria-modal, labelled "Incoming voice|video call from <caller>".

Found while verifying #187 DP6 (decline still dismisses when the decline
send fails — it does).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/components/CallEmbedProvider.tsx

@@ -180,7 +180,15 @@ function IncomingCall({ dm, info, onIgnore, onAnswer, onReject }: IncomingCallPr
             escapeDeactivates: false,
           }}
         >
-          <Dialog style={{ maxWidth: toRem(324) }}>
+          <Dialog
+            style={{ maxWidth: toRem(324) }}
+            // #187: the overlay had no role or name, so a screen reader landed
+            // on an unlabelled button with no hint that a call was ringing.
+            // Match the in-call banner (role="alert" + label).
+            role="alertdialog"
+            aria-modal="true"
+            aria-label={`Incoming ${info.intent === 'video' ? 'video' : 'voice'} call from ${getMemberName(info.room, info.sender) ?? info.sender}`}
+          >
             <Box style={{ padding: config.space.S400 }} direction="Column" gap="700">
               <Text size="T200" align="Center">
                 {getMemberName(info.room, info.sender)}