feat(linux): add Flatpak CI, .deb builder, and AUR PKGBUILD

[cozyGarage]

Summary

• Add `.github/workflows/flatpak-linux.yml` to build `linux/flatpak/com.tablepro.linux.json` with `flatpak-builder` on GNOME 47.
• Vendor GtkSourceView 5.16 into the Flatpak manifest; keep `scripts/build-flatpak.sh` and `scripts/generate-cargo-sources.sh` for local / Flathub offline flows.
• Add Debian packaging under `linux/packaging/debian/` plus `scripts/build-deb.sh` (produces a rootless `.deb`).
• Add an AUR `PKGBUILD` and packaging README. Update AppStream metainfo for MSSQL / ClickHouse.

Test plan

• `python3 -c 'import json; json.load(open("flatpak/com.tablepro.linux.json"))'`
• `./scripts/build-deb.sh` → `packaging/out/tablepro_0.1.0-1_amd64.deb` (6.5 MB) with expected Depends
• GtkSourceView tarball sha256 verified against gnome.org
• CI Flatpak job green on this PR
• Optional local: `./scripts/build-flatpak.sh` after installing flatpak runtimes

Made with Cursor

[cozyGarage]

I have read the CLA Document and I hereby sign the CLA.

[datlechin]

Hi @cozyGarage, could you please resolve the conflict with linux branch?