Steam launch options: accept env-style args without %command% (all stores)

[Leb-Sun]

Fixes the %command% requirement Nick raised: a plain env-style launch option like
WINEDLLOVERRIDES=D3D12=d or DXVK_HUD=FPS now applies without appending %command%,
and behaves consistently across Steam / Epic / GOG / Custom.

What changed (2 files, +40/−1):

• SteamLaunchOptions.parse() — with no %command%, a leading run of UPPERCASE KEY=VALUE
  tokens is read as env vars (shell-style prefix); the first non-env token onward stays game
  args, kept verbatim (quotes preserved). The %command% branch is untouched, so existing
  X=Y %command% args strings behave exactly as before.
• XServerDisplayActivity — Epic/GOG/Custom now route their exec args through
  SteamLaunchOptions.gameArgs() like the Steam path already does, so %command%/env tokens
  stop leaking into the game's argv (cross-store parity).

Behavior:

• WINEDLLOVERRIDES=D3D12=d, DXVK_HUD=FPS, DXVK_HUD=FPS -windowed → env applied, no %command%.
• -dx11, +connect, lowercase key=value → stay game args (uppercase-guarded, so real custom
  args aren't swallowed).
• WINEDLLOVERRIDES="winhttp=n,b" %command% (pasted mod/protondb command) → unchanged.

Known edge (accepted): an UPPERCASE KEY=VALUE that is genuinely a game argument, typed
with no %command%, would be read as env — rare; %command% KEY=val forces it back to an arg.

Cut from upstream main; opened against my own main to trigger a build and track it separately
(no upstream PR yet).