feat: show the app icon on Wayland without AppImage integration

[Lolle2000la]

Problem

On Wayland the window icon is not a client property: compositors resolve it (and the app's display name / launcher grouping) through the desktop entry whose file name matches the toplevel's app_id. An AppImage only gets such an entry when a desktop-integration tool (appimaged, AppImageLauncher, Gear Lever, ...) has "installed" it, so an unintegrated AppImage always shows a generic icon even though it ships MediaSort.desktop and the icon files itself. The old app-id logic (appimagekit-<stem>.desktop / <stem>) only covered the naming schemes of some integration tools and can't help on its own.

The xdg-toplevel-icon-v1 protocol would solve this properly, but iced 0.14 pins winit 0.30.13, which cannot speak it (winit only gained support on its main branch, rust-windowing/winit#4288). Until iced moves to a newer winit, the standard cross-compositor route is to make the desktop-entry lookup succeed.

Solution

New crates/media-sort-gui/src/linux_desktop.rs, called once from main.rs to produce the window's application_id:

1. Adopt a canonical entry. If a packaged or hand-written MediaSort.desktop (the canonical id) exists anywhere, it wins: its name is used as the app id and nothing is written.
2. Adopt an integration entry. Otherwise it scans $XDG_DATA_HOME/applications and $XDG_DATA_DIRS/*/applications for a *.desktop file whose Exec/TryExec launches the running $APPIMAGE (or, when there is no AppImage, the running executable) - any integration tool, any file name; quotes, env prefixes and \s escapes are parsed and symlinked AppImages are resolved. Entries inside the running AppImage's own $APPDIR mount are ignored (the compositor cannot see the mount, so they must not count as "already registered"). If found, its file name is adopted as the app id and nothing is written.
3. Self-provision when missing. Otherwise a hidden (NoDisplay=true) desktop entry plus the scalable MediaSort icon are written into $XDG_DATA_HOME, under a name unique to this module (applications/mediasort-wayland.desktop and icons/hicolor/scalable/apps/mediasort-wayland.svg).

The provisioned files are kept between runs - they are what gives the shell the icon, and removing them at exit would only churn icon/menu caches and break window grouping across a restart. Because their name is unique to this module, a self-provisioned file can never collide with or shadow a packaged, distro- or tool-managed MediaSort.desktop; a foreign file found at one of those paths is left untouched. They are removed again only when a canonical MediaSort.desktop appears (or when one was written under the canonical name by an earlier revision), so the packaged entry's icon and running state take over. Writes go through path_utils::atomic_write and are skipped when content is unchanged.

Nothing is provisioned on non-Wayland sessions: on X11 the icon is delivered by the window itself (window::Settings.icon), so only the app id is set there - to the canonical MediaSort, which matches the packaged and integration entries' StartupWMClass=MediaSort. This also gives cargo run and the standalone binary an icon on Wayland (without an AppImage, Exec points at the running executable).

Paths that cannot be represented portably (control characters, %, $, backticks) are skipped, so an invalid desktop entry is never written; spaces, quotes and backslashes are escaped for both the key-file and the shell parsing layers (verified against desktop-file-validate, GLib's loader and KDE's KConfig/KShell).

The embedded icon is packaging/linux/icon.svg itself - the scalable SVG the AppImage also ships - so it cannot drift from the packaged icon. GNOME (librsvg) and KDE (QtSvg) render theme SVGs natively (Breeze ships SVG-only icons), so no raster renditions are needed.

Testing

• 26 unit tests: adoption by Exec/TryExec/symlink/env wrapper/plain executable/indented Exec, provisioning incl. the hidden-entry contents and the icon file, refresh when the executable moves, persistence and repair across runs, never overwriting a foreign entry or a user-replaced icon, removal when a canonical entry appears, canonical-name migration, quote/escape handling, rejection of unrepresentable characters, $XDG_DATA_DIRS parsing and the $APPDIR exclusion.
• Exec escaping verified against desktop-file-validate, GLib's actual GDesktopAppInfo loader and KDE's KConfig/KShell.
• Manually verified on KDE Wayland: WAYLAND_DEBUG=1 shows xdg_toplevel.set_app_id(...) and the provisioned files appear under $XDG_DATA_HOME; desktop-file-validate passes on the generated entry.
• cargo fmt --all -- --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace.

Notes

• The maintainer's existing hand-written ~/.local/share/applications/media_sort.desktop is adopted as-is when that AppImage runs (its Exec matches), so no duplicate hidden entry is created for it.
• xdg-toplevel-icon-v1 is only partially implemented across compositors (KWin yes, Mutter/GNOME currently no), so even after iced ships a newer winit the desktop-entry route remains necessary for GNOME. When iced does, this module can be reduced to that protocol or removed - noted in AGENTS.md.