Fix Linux pet taskbar and close regressions without losing chat focus

[alvinunreal]

Summary

• Keep X11/XWayland pet windows focusable for chat and plugin inputs, and request taskbar, pager, and supported KDE switcher exclusion after the actual map event. Preserve other window-manager state and apply the hints across default, agent, plugin, and LAN pet show paths.
• Treat window-manager close of the default pet as a temporary hide, preserving its launch preference and reusable window; keep explicit Hide persistent. Refresh the tray when visibility actually changes.
• Select the default X11 backend before Electron initializes Ozone. A bounded, sandbox-safe bootstrap supports packaged/AppImage launches and supervises unpackaged development processes; the native-Wayland opt-out remains available.
• Add behavior tests, package contract checks, and updated desktop/development docs. Record the approved read-only dependency source revisions used in the investigation.

Verification

• pnpm --filter @open-pets/desktop check, pnpm docs:build, git diff --check.
• Packaged arm64 KDE/X11 VM: first-show skip states, visible Alt+Tab switcher excludes the pet, real chat typing, WM close → Show Default Pet in tray → same-window re-show; launch preference remains true.
• Packaged arm64 GNOME/Wayland VM: unflagged launch reaches XWayland with real XID and standard skip states; OPENPETS_ALLOW_WAYLAND=1 remains native Wayland. After clearing first-run/keyring dialogs, a captured GNOME Alt+Tab switcher listed Files and Terminal but not the visible pet, and a screenshot confirmed typed text in pet chat.
• Mounted arm64 AppImage on KDE: startup and skip states. Linux dev:control-center over SSH on GNOME: X11 startup and coordinated Electron/Vite shutdown.

DEB, RPM, tar.gz, and x86_64 artifacts were not run end-to-end in these VMs.

Fixes #233. Keeps #228/#227 keyboard focus behavior.

[alvinunreal]

Update: PR #234 is merged at 4d5d0dd393e18cbe983d5cfa1b32b7bf3b2b1ce9. Its post-merge Linux x64 CI run passed and the four resulting Linux assets now replace the Linux downloads on the original v4.0.0 GitHub release. The complete checksum manifest was updated, macOS/Windows assets remained unchanged, and the separate Linux-only rebuild release and tag were removed.