chore: stop tracking build/ (compile artifacts) + add .gitignore

build/ is generated by CMake and must not be in the repo; it bloated .git to
571MB and made pushes fail (gitea/nginx resets the connection on large uploads).
Untrack the 397 build/ files (kept on disk, just removed from the index) and add
a .gitignore covering build/ and build_*/ so they stay out going forward.
Repo is now source-code only and pushes are small.

.gitignore

@@ -0,0 +1,20 @@
+# Build directories (never commit) - recreated by CMake
+build/
+build_*/
+build_linux/
+build_win/
+build_mailio/
+build_asan/
+CMakeCache.txt
+CMakeFiles/
+
+# Qt / IDE local settings
+.qtcreator/
+*.user
+
+# Test binaries / temp
+/test_*
+
+# OS files
+.DS_Store
+Thumbs.db