fix(a11y): 36 modal dialogs announce themselves and take focus (#185)

Most modals rendered a folds Dialog/Modal with no role and no name, and
their focus traps used `initialFocus: false`, so focus stayed behind the
modal and a screen reader never announced it.

- 32 dialogs with a visible heading: role="dialog", aria-modal,
  aria-labelledby → the heading (given an id), tabIndex=-1.
- 4 dialogs that already had a name (Leave Room, room topic viewer,
  server ACL, room-nav prompt): role + aria-modal.
- Their focus traps drop `initialFocus: false` for focus-trap's default
  (keep an already-focused autoFocus field, else the first tabbable
  element) with the dialog itself as fallbackFocus, so a dialog without
  a tabbable node can't crash the trap. Traps that live in a parent
  (UIA stages, Logout, Forward, Invite) get the semantics only.
- The file drop overlay is deliberately left alone (not a dialog).

Checked at runtime: Join with Address, Delete Message, Report Message,
Leave Room and Logout open as named dialogs with focus inside and close
with Escape (Tab first when a text field has focus — the shared
stopPropagation keeps Escape from discarding typed text, by design).
The axe e2e spec (6 tests) passes; eslint warnings unchanged (46).
17 modals with no heading (image/file viewers, loading screens) remain.

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

src/app/components/DeviceVerification.tsx

@@ -365,15 +365,23 @@ export function DeviceVerification({ request, onExit }: DeviceVerificationProps)
       <OverlayCenter>
         <FocusTrap
           focusTrapOptions={{
-            initialFocus: false,
+            fallbackFocus: '#deviceverification-dialog-1',
             clickOutsideDeactivates: false,
             escapeDeactivates: false,
           }}
         >
-          <Dialog variant="Surface" style={modalStyle}>
+          <Dialog
+            id="deviceverification-dialog-1"
+            role="dialog"
+            aria-modal="true"
+            aria-labelledby="deviceverification-dialog-1-title"
+            tabIndex={-1}
+            variant="Surface"
+            style={modalStyle}
+          >
             <Header style={DialogHeaderStyles} variant="Surface" size="500">
               <Box grow="Yes">
-                <Text as="h2" size="H4">
+                <Text id="deviceverification-dialog-1-title" as="h2" size="H4">
                   Device Verification
                 </Text>
               </Box>