Fix int64 -> int64_t in remaining test files, finalize Flatpak manifest

- bignum_tests, script_tests, util_tests, wallet_tests: int64 -> int64_t
- Flatpak manifest: use GitHub URLs instead of local paths (Flathub-ready)
- Add flathub.json (x86_64 only)
- Fill SHA256 hashes for static assets

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/test/util_tests.cpp

@@ -199,7 +199,7 @@ BOOST_AUTO_TEST_CASE(util_FormatMoney)
 
 BOOST_AUTO_TEST_CASE(util_ParseMoney)
 {
-    int64 ret = 0;
+    int64_t ret = 0;
     BOOST_CHECK(ParseMoney("0.0", ret));
     BOOST_CHECK_EQUAL(ret, 0);