build: bump Node 20 to Node 24 across CI workflows and Flatpak manifest

Cherry-picked from PR #377 (thanks @Mar0xy) — Node-only portion, no
Discord IPC additions:

- 4 workflows (release-{linux,linux-aarch64,macos,flatpak}.yml) now
  install Node 24
- packaging/flatpak/com.blitzfc.qbz.yml: SDK extension node20 → node24,
  build append-path and npm_config_nodedir aligned

Forcing function: @icons-pack/svelte-simple-icons@7.x already in use
declares engines.node >=24, so Node 20 produces an EBADENGINE warning
in CI. Node 20 also reached EOL on 2026-04-30. Node 24 has been LTS
since October 2025 (~7 months of maturity).

Discord IPC permissions and qbz-wrapper script from #377 were
intentionally NOT cherry-picked — there's no Discord RPC feature in
QBZ today (#336 is still planned but not implemented), so adding the
sandbox permission and the launch wrapper now would expand the
attack surface for nothing. They'll land together with the actual
feature when it ships.

cc

.github/workflows/release-flatpak.yml

@@ -94,7 +94,7 @@ jobs:
             org.gnome.Sdk//49
           # Install SDK extensions - flatpak-builder will auto-resolve the correct version based on the SDK
           flatpak install --user -y --noninteractive flathub \
-            org.freedesktop.Sdk.Extension.node20//25.08 \
+            org.freedesktop.Sdk.Extension.node24//25.08 \
             org.freedesktop.Sdk.Extension.rust-stable//25.08 \
             org.freedesktop.Sdk.Extension.llvm20//25.08