flathub: build from source instead of the prebuilt release zip

Part of #1060. Reviewer feedback on the submission (flathub/flathub#10606): "Any particular reason this is not being built from source?"

Problem

The manifest downloads Querya-Desktop-<version>-linux.zip from the GitHub release and installs the binaries from it. Flathub rules (requirements):

"All source available submissions must be built entirely from source code."
"Binary or precompiled files must not be present in the submission pull request."
exceptions are case by case and for well-known vendors, so we should not count on one

Build the app with flatpak-builder from the tagged source (Flutter SDK, Dart packages and the Rust/native parts offline, as Flathub builds have no network)
Look at how other Flutter apps on Flathub do it (the community flatpak-flutter generator is the usual starting point; verify that it fits our Flutter version)
Decide what to do with the vsync-patched engine (QueryaHub/flutter-engine-linux-120hz): it is a prebuilt .so today; either build it from source in the manifest or ship the stock engine for Flathub and document the 60 fps cap (perf(linux): app is capped at 60 fps because the GTK embedder has no vsync #980)
Pin every source (git tag + commit, checksums for archives)
Verify with flatpak-builder --install and run the app

The manifest must be written by hand: Flathub's policy forbids AI-generated or AI-assisted manifests (see the policy issue linked from the epic).