fix(build): remove SODIUM_USE_PKG_CONFIG from .cargo/config.toml [env] block

Cargo injects [env] entries directly into build script environments,
bypassing shell-level unset and env -u entirely. SODIUM_USE_PKG_CONFIG
was set to "0" in [env], which libsodium-sys-stable build.rs treats as
"pkg-config enabled" (env::var().is_ok() returns true for any value
including "0"). Combined with SODIUM_LIB_DIR set in the workflow, this
triggered the incompatibility panic on every build.

The original comment claiming this "avoids memset_explicit on Windows"
was incorrect — SODIUM_USE_PKG_CONFIG controls detection strategy, not
link behaviour. The actual Windows memset_explicit fix is handled via
SODIUM_LIB_DIR pointing to pre-built MinGW sodium.

src-tauri/.cargo/config.toml

@@ -9,7 +9,4 @@ rustflags = ["-C", "link-arg=-Wl,--exclude-all-symbols"]
 # Use system OpenSSL instead of vendoring from source (which requires Perl modules
 # unavailable on some environments and breaks clippy/check).
 OPENSSL_NO_VENDOR = "1"
-
-# Force libsodium to use minimal mode which avoids memset_explicit on Windows
-SODIUM_USE_PKG_CONFIG = "0"
 SODIUM_STATIC = "1"