feat(install): write launcher language marker files

Some games include a language.txt marker in the unpacked local tree, similar
in spirit to account_name.txt. Installs and updates now carry the launcher
language alongside the account name so those game-provided marker files are
rewritten before staged files are promoted into local/.

The Tauri command boundary keeps the UI setting vocabulary as de/en, then maps
it to the file vocabulary expected by games: german or english. Unknown values
continue through the existing DEFAULT_LANGUAGE path, so the marker file falls
back to english just like script launch arguments fall back to en.

The transaction layer deliberately reuses the same first-match traversal helper
for both marker files. The searches stay independent, so games may place
account_name.txt and language.txt in different directories if their archive
layout requires that.

Test Plan:
- just fmt
- just test
- just frontend-test
- just clippy
- deno task build
- git diff --check

Refs: none

crates/lanspread-peer/ARCHITECTURE.md

@@ -116,6 +116,9 @@ Downloaded and installed are independent predicates:
 - `installed` is true when `<game_root>/local/` is a directory. The contents of
   `local/` are user-owned and are skipped by manifests, fingerprints, and file
   serving.
+- Install and update transactions unpack into staging, then overwrite the first
+  discovered game-provided `account_name.txt` and `language.txt` files under
+  the staged tree from launcher settings before promoting it to `local/`.
 
 Reserved per-game paths: