build_all: incremental build + per-step toolchain install

Skip steps whose dist/ artifact already exists; add --clean/-c to
force a full rebuild. Install sphinx/linuxdoc, build, pyinstaller
in their respective steps instead of upfront. Auto-add flathub
remote and install missing Flatpak SDK/runtime deps before step 4.

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

src/VERSION

@@ -1 +1 @@
-0.1.2
+0.1.3