Improve compact chat accessibility and message selection (#57)

[trufae]

The remaining Linux feedback in #57 identifies overly verbose compact rows, hard-to-find message selection, and an application ID spoken in GNOME's switcher.

• Refresh chat-row accessible names when toggling compact mode, omitting the hidden timestamp and preview while retaining the name, unread count, requests, mentions, pinned and muted status.
• Add Space to select a focused message, rename the menu action to “Select Messages” with its shortcut, and focus the selected message's labelled checkbox. Subsequent Space presses toggle selection; embedded controls retain their normal key handling. Cancel restores focus.
• Use “Parla” for the X11 WM_CLASS fallback and the matching StartupWMClass. Keep the application ID unchanged for Wayland, D-Bus and installed desktop integration.
• Document the selection workflow and the desktop installation needed for GNOME to resolve a Wayland application ID.

The naming change follows GNOME Shell's fallback to WM_CLASS and GTK's separate Wayland application identity. An uninstalled Wayland binary or AppImage can still display its application ID until its desktop launcher is installed.

Validation: both accessibility-enabled and -Da11y=false builds pass with warnings treated as errors (-Dwebxdc=false locally), and all 14 existing Meson tests pass in each configuration. No new UI test is claimed. This environment cannot run a display server, so interactive keyboard, Orca, NVDA and GNOME switcher checks remain pending.

Suggested checks: toggle the sidebar compact/full while a chat row is focused; use Space and the message menu to enter selection, toggle multiple checkboxes and cancel; confirm Enter still replies; check Alt+Tab for an installed Wayland build and an uninstalled X11 build.

Related to #57. Kept open for screen-reader confirmation and remaining Wayland desktop-integration reports. Based directly on main and separate from the #80 fix in #82.