feat(rendering): support the disable-hardware-acceleration toggle on Linux

WebKitGTK has no Chromium command line, so the startup override now
dispatches per platform: Windows keeps appending `--disable-gpu` to
`WEBVIEW2_ADDITIONAL_BROWSER_ARGUMENTS`, while Linux sets
`WEBKIT_DISABLE_DMABUF_RENDERER=1` and `WEBKIT_DISABLE_COMPOSITING_MODE=1`
unless the user already exported them. The settings section is gated on
Windows or Linux (macOS/WKWebView still exposes no knob), and the
description is updated across all 10 locales.

Two defects in the existing startup override are fixed along the way,
both reachable from the UI's own "Restart now" button:

- The override ran after `init_desktop()`, which spawns a
  `tracing_appender` worker thread. `set_var` is UB once another thread
  exists, so it now runs as the first statement of `run()` and the
  functions are `unsafe fn` to make the precondition part of the
  contract.
- `tauri::process::restart` spawns the replacement process without
  clearing the environment, so an injected override was inherited across
  a restart and a launch that read the preference as `false` left it in
  place — turning the toggle back off never took effect. Injected keys
  are now recorded in `CODEG_WEBVIEW_RENDERING_OVERRIDE` and withdrawn on
  the next launch, which also keeps values the user exported themselves
  from being clobbered.