Let the launcher find an icon for the window

The icon was already embedded and had been all along: `app.slint` sets it and
`build.rs` compiles it into the binary with `EmbedFiles`. That is not what a
Wayland compositor looks at. It ignores a client-set icon entirely, matches
the surface's `app_id` against installed `.desktop` files, and takes the icon
from the one whose basename agrees. Nothing set an app id and no desktop entry
existed, so GNOME had nothing to resolve and drew the placeholder.

Both halves are needed and neither substitutes for the other: the embedded
icon is what X11 and the window itself use, and the desktop entry is what the
overview, the dash and alt-tab use.

The app id, the `.desktop` basename and the installed icon's filename are one
string in three places — `paris.tourolle.darkroom`, the same reverse-DNS name
the Android manifest already uses, because it is one application. Change one
without the others and the icon disappears again with nothing logged, so each
file says so where the string appears.

The PKGBUILD builds from the checkout rather than a release tarball, so
`makepkg -si` installs what is being worked on. Install verified by inspection
of the built package: binary, desktop entry, and the icon under
hicolor/256x256 by the name the entry asks for.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -2,3 +2,12 @@
 /target-android
 Cargo.lock.bak
 *.log
+
+# makepkg build products. `packaging/PKGBUILD` and the .desktop entry are
+# sources and belong in the tree; everything makepkg derives from them does
+# not — `pkg/` and `src/` are staging directories it recreates on every run,
+# and the package itself is 33 MB of compiled output.
+/packaging/pkg/
+/packaging/src/
+/packaging/*.pkg.tar.*
+/packaging/*.log