feat(install): make Flatpak the default one-liner pathway

[hikaps]

Problem

We agreed Flatpak is the default GUI (it runs on every distro; the native binary fails on Arch/SteamOS via copy-relocation). But the one-liner (install.sh) contradicted this: on every non-SteamOS distro it ran install_tarball(), installing the native couchplay binary + helper. Only the SteamOS (install_sysext) path actually installed the Flatpak.

So a user running the documented one-liner on Arch/Fedora/Bazzite got the native binary — the exact thing that breaks on Arch/SteamOS.

Change

• New install_flatpak() pathway is the default for all non-SteamOS distros:
  1. Download + checksum-verify couchplay.flatpak.
  2. Ensure the Flathub remote + org.kde.Platform 6.10 runtime.
  3. flatpak install --user the bundle as the invoking user (Flatpak never runs as root; everything user-scoped goes through sudo -u "$REAL_USER").
  4. Export the privileged helper from the Flatpak (flatpak run … install-helper.sh export) and install it system-wide — the same path documented in the README, which install-helper.sh already detects (flatpak-export layout, line 89-92).
• main() pathway selection: SteamOS → sysext; --tarball → native (Fedora-family opt-in); default → flatpak. New --flatpak flag to force it.
• flatpak.yml: generate + publish couchplay.flatpak.sha256 (beta and stable releases) so the one-liner can verify the bundle — previously only the tarball/sysext had checksums. Old releases without the sha are handled gracefully (verification skipped).
• README: surface the one-liner as Quick install (Flatpak GUI + helper in one command); the manual flatpak/helper steps remain below it.

Verification

• bash -n scripts/install.sh clean; shellcheck reports zero new findings (only the pre-existing binary_name SC2034).
• flatpak.yml parses as valid YAML.
• Confirmed install-helper.sh detects the flatpak-export layout ($SCRIPT_DIR/couchplay-helper + $data/), so the export→install step resolves correctly.
• Stable releases already ship couchplay.flatpak (via flatpak.yml's v* tag trigger); this PR only adds the checksum.

Notes

• No C++ changes; the unit suite is unaffected.
• The stable main one-liner stays on the old installer until the next release cut; this lands on develop (beta) first, as expected.
• End-to-end flatpak install can't be exercised in CI (needs a real runtime download + bwrap); validated by syntax/lint + structural review following the documented README path.