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/message/content/FileContent.tsx

@@ -105,13 +105,18 @@ export function ReadTextFile({ body, mimeType, url, encInfo, renderViewer }: Rea
           <OverlayCenter>
             <FocusTrap
               focusTrapOptions={{
-                initialFocus: false,
+                fallbackFocus: '#file-viewer-2-dialog',
                 onDeactivate: () => setTextViewer(false),
                 clickOutsideDeactivates: true,
                 escapeDeactivates: stopPropagation,
               }}
             >
               <Modal
+                id="file-viewer-2-dialog"
+                role="dialog"
+                aria-modal="true"
+                aria-label={`File: ${body}`}
+                tabIndex={-1}
                 className={ModalWide}
                 size="500"
                 onContextMenu={(evt: React.MouseEvent) => evt.stopPropagation()}
@@ -194,13 +199,18 @@ export function ReadPdfFile({ body, mimeType, url, encInfo, renderViewer }: Read
           <OverlayCenter>
             <FocusTrap
               focusTrapOptions={{
-                initialFocus: false,
+                fallbackFocus: '#file-viewer-1-dialog',
                 onDeactivate: () => setPdfViewer(false),
                 clickOutsideDeactivates: true,
                 escapeDeactivates: stopPropagation,
               }}
             >
               <Modal
+                id="file-viewer-1-dialog"
+                role="dialog"
+                aria-modal="true"
+                aria-label={`File: ${body}`}
+                tabIndex={-1}
                 className={ModalWide}
                 size="500"
                 onContextMenu={(evt: React.MouseEvent) => evt.stopPropagation()}