Drop legacy PSLauncher.exe backward compat (not in prod yet)

Simplifies the rename done in ce3979d : PS_Launcher.exe is the only
name. Removes the dual-path lookup in LauncherSelfUpdater, the
duplicated taskkill blocks in the .bat scripts, the legacy patterns
in .gitignore, and the explanatory comments about the migration.

Cleaner code, single source of truth for the binary name.

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

.gitignore

@@ -53,11 +53,6 @@ server/builds/*
 .claude/
 
 # Binaires copiés par le post-publish à la racine du repo
-# (legacy : PSLauncher.exe — pre-v0.14 ; nouveau : PS_Launcher.exe — depuis v0.14)
-/PSLauncher.exe
-/PSLauncher.Updater.exe
-/PSLauncher-*.exe
-/PSLauncher*.bak
 /PS_Launcher.exe
 /PS_Launcher.Updater.exe
 /PS_Launcher-*.exe
@@ -68,7 +63,6 @@ server/builds/*
 
 # WebView2 user data folder créé à côté de l'exe au runtime (cache, cookies,
 # sync data Edge Chromium). Peut faire plusieurs Mo.
-/PSLauncher.exe.WebView2/
 /PS_Launcher.exe.WebView2/
 
 # Fichiers de lock Office (Word/PowerPoint) commençant par ~$