chore(pass-desktop): add Flatpak desktop entry and AppStream metainfo

[the-narwhal]

Add Flatpak desktop entry and AppStream metainfo for Proton Pass desktop

What

Adds two files to applications/pass-desktop/assets/:

• me.proton.Pass.desktop — XDG desktop entry for the Linux app
• me.proton.Pass.metainfo.xml — AppStream component metadata for Flathub/GNOME Software

Why

The Flathub listing for Proton Pass (me.proton.Pass) is being migrated
from a binary .deb wrapper to a full build from source. Flathub's
enhanced requirements mandate that GPL-licensed apps build from source.

Both files are currently carried as copies inside the Flathub manifest
repository. Having them here in the upstream source tree means:

• Future Flatpak builds can reference them directly from the git checkout
• The metadata stays in sync with the app automatically on each release
• Proton has direct ownership and control over what ships to Flathub/GNOME Software

How the files were derived

me.proton.Pass.desktop — all values taken directly from forge.config.ts
(MakerDeb/MakerRpm options: productName, genericName, productDescription,
categories, homepage) and the packager config (executableName, appBundleId).

me.proton.Pass.metainfo.xml — written to the
Flathub AppStream guidelines.
Description text is taken from the existing Proton Pass marketing copy.
Release notes are sourced from CHANGELOG.md.

Once merged

The type: file sources in the Flathub manifest will be updated to
reference these paths directly from the upstream git checkout, and the
local copies in the manifest repository will be removed.

Related Flathub PR: flathub/me.proton.Pass#99