launcher: persist the hook DLL override in the prefix, not in launch options

Wine ignores the game-directory version.dll proxy unless an override names it.
WINEDLLOVERRIDES covers only a process the launcher spawns itself, so the documented
fallback was to have the user paste Steam launch options by hand - a step a normal
player cannot be expected to perform, and the reason the game had to be started
through a specific wrapper at all.

The launcher now persists version=native,builtin into the prefix registry via Wine
own reg tool before launching (ensure_dll_override). It is /f-idempotent, so it runs
on every launch and repairs a prefix the player has reset or replaced, and it applies
to EVERY launch path including Steam Play. This mirrors what BepInEx documents for
Proton (configure the proxy in winecfg rather than the environment) and what Proton
already does in this prefix for other titles. Best-effort: a failure is reported in
plain language and the launch still carries WINEDLLOVERRIDES.

STEAM_LAUNCH_OPTIONS is demoted to a fallback for prefixes we have never prepared.

Also fixes a pre-existing clippy manual_is_multiple_of in app.rs that was failing the
strict lint gate. fmt clean, clippy -D warnings clean, 75 tests pass.

src/app.rs

@@ -2177,7 +2177,7 @@ fn group_thousands(digits: &str) -> String {
     let len = bytes.len();
     let mut out = String::with_capacity(len + len / 3);
     for (i, b) in bytes.iter().enumerate() {
-        if i > 0 && (len - i) % 3 == 0 {
+        if i > 0 && (len - i).is_multiple_of(3) {
             out.push(',');
         }
         out.push(*b as char);