fix: drop extern "C" on main to clear -Wpedantic warning

src/main.cpp:161:16: warning: cannot declare '::main' with a linkage
specification [-Wpedantic]

Long-standing upstream OpenInverter code (predates SKUDAK). C++ forbids
linkage specifications on main; with -pedantic in CPPFLAGS this fires
every build.

Confirmed safe to drop:
- libopencm3 vector.c:35 forward-declares int main(void) with default
  linkage and invokes it at boot.
- Linker resolves main by symbol name, not by linkage attribute.
- stm32-sine has the same upstream issue; same fix applies if we ever
  push upstream.

Bump VERSTR -S3 -> -S4 so the OI web UI version field shows the new
build. Verified in the built binary as "4=1.20.R-S4".

Build: clean, no compiler warnings (only the long-standing linker RWX
notice remains, unrelated).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -4,3 +4,4 @@ stm32_charger*
 linker.map
 *.layout
 *.out
+.DS_Store