Fix black window on Linux + NVIDIA (WebKitGTK DMABUF renderer)

On the NVIDIA proprietary driver the app booted, logged "miniSCP started",
reported no error — and painted a completely black window, with stderr showing
"Failed to create GBM buffer of size WxH: Invalid argument" and an X11
GLXBadWindow. WebKitGTK's DMABUF renderer cannot allocate buffers there;
WEBKIT_DISABLE_DMABUF_RENDERER=1 is the upstream workaround. Pre-existing on
Linux, unrelated to the context-menu change — it just had never been looked at
because nothing on Linux had been run before.

The app CANNOT apply this to itself. WebKit reads the variable when it forks
its web process, and Bun's `process.env` writes do not propagate to child
processes (verified: a spawned child sees it unset), so setting it in
src/bun/index.ts is inert even from a module imported before `electrobun/bun`.
It has to be in the environment before the app process starts, which means
every launch path must inject it:

- scripts/linux-gpu.ts — shared `launchEnv()` helper, gated on
  /proc/driver/nvidia so Intel/AMD keep hardware compositing, and never
  overriding an explicit value from the user's environment.
- scripts/dev.ts — uses it for `bun run dev`.
- scripts/start.ts (new) — `bun run start` previously called `bunx electrobun
  dev` straight from package.json, where no env could be set; it now goes
  through a script like dev does.
- scripts/release.ts — writes a `miniSCP.sh` wrapper beside bin/launcher in the
  Linux tarball, since a packaged app goes through none of the above.

Documented in gotchas.md (including that a "does it boot?" log check does NOT
catch this — the log looks healthy) and platform-notes.md, with the smoke test
updated to require that the UI actually renders.

Verified: backend `tsc --noEmit` clean, `bun run build:ui` clean, and
`bun run start` now boots with zero GBM/GLX errors.

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -119,7 +119,8 @@ bun run release -- --target linux-x64   # on a Linux x64 machine
 Artifacts land in `release/<target>/miniSCP-Build<n>-<target>.*` — a `.dmg`
 per mac arch; on Windows a `-Setup.zip` installer **and** a `-portable.zip`
 (unzip anywhere and run); on Linux a portable `.tar.gz` (unpack and run
-`bin/launcher`). The mac-x64 build fetches electrobun's x64 CLI once
+`miniSCP.sh`, which sets the NVIDIA workaround then execs `bin/launcher`).
+The mac-x64 build fetches electrobun's x64 CLI once
 into `~/.cache/miniscp/` and runs it under Rosetta so the whole pipeline
 believes it is on an Intel Mac. CI: `.gitea/workflows/release.yml` runs the
 same script on a macos + windows runner matrix once act_runners are
@@ -259,10 +260,11 @@ its tray icon.
 - **Tray visibility** depends on the desktop: KDE, XFCE, MATE and Cinnamon show
   it; GNOME 3.26+ needs the AppIndicator extension. The app works either way.
 - **Unsigned and unpackaged**: no AppImage, `.deb`, desktop entry or icon
-  installation — unpack the tarball and run `bin/launcher`.
-- Harmless startup warnings (`GLXBadWindow`, `Failed to create GBM buffer`, an
-  appindicator deprecation notice) are documented in
-  [gotchas](docs/gotchas.md#linux).
+  installation — unpack the tarball and run `miniSCP.sh`.
+- **NVIDIA GPUs** need `WEBKIT_DISABLE_DMABUF_RENDERER=1` or the window renders
+  black. Every launch path (dev scripts and the packaged `miniSCP.sh` wrapper)
+  sets it automatically when an NVIDIA driver is detected — see
+  [gotchas](docs/gotchas.md#linux) if you add a new way to launch the app.
 
 Full dependency table, diagnostics and the first-boot smoke test:
 [platform notes](docs/platform-notes.md).