fix(a11y): name the headless modals too — Seen by, source, file viewers (#185)

Follow-up to ce8ed89f for the modals that have no visible heading to
point at: the three "Seen by" reader lists (read-receipt pill, "is
following", message menu), View source, the text/PDF file viewers, the
room-card join error and the user-profile modal get role="dialog",
aria-modal and an aria-label; their traps move focus in (fallbackFocus
on the dialog) where the trap is local.

Verified: the receipt pill opens a "Seen by" dialog with focus on
Close; Escape closes it and focus returns to the pill.

The remaining unnamed Modal/Dialog uses are startup/loading and error
screens (config, feature check, spec versions, client root, password
reset) and wrappers around components that carry their own role (image
viewer).

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

src/app/components/UserRoomProfileRenderer.tsx

@@ -71,7 +71,15 @@ function UserRoomProfileContextMenu({ state }: { state: UserRoomProfileState })
       <Overlay open backdrop={<OverlayBackdrop />}>
         <OverlayCenter>
           <FocusTrap focusTrapOptions={focusTrapOptions}>
-            <Modal size="500" style={MOBILE_FULLSCREEN}>
+            <Modal
+              id="user-profile-dialog"
+              role="dialog"
+              aria-modal="true"
+              aria-label="User profile"
+              tabIndex={-1}
+              size="500"
+              style={MOBILE_FULLSCREEN}
+            >
               {/* Full-screen covers the backdrop (no tap-to-dismiss) and the
                   profile has no self-close, so provide an explicit close. */}
               <Header size="600" style={{ flexShrink: 0, paddingRight: config.space.S200 }}>