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>

scripts/start.ts

@@ -0,0 +1,20 @@
+// `bun run start`: launch the app against the prebuilt Angular bundle (no ng
+// serve / hot reload — that's `bun run dev`). Exists as a script rather than a
+// bare `bunx electrobun dev` in package.json so the Linux GPU workaround can be
+// injected into the environment before the app process starts.
+import { launchEnv } from "./linux-gpu";
+
+const app = Bun.spawn(["bunx", "electrobun", "dev"], {
+	stdout: "inherit",
+	stderr: "inherit",
+	env: launchEnv(),
+});
+
+function shutdown(): void {
+	app.kill();
+	process.exit(0);
+}
+process.on("SIGINT", shutdown);
+process.on("SIGTERM", shutdown);
+
+process.exit(await app.exited);