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/linux-gpu.ts

@@ -0,0 +1,33 @@
+// PORTABILITY: WebKitGTK's DMABUF renderer cannot allocate GBM buffers on the
+// NVIDIA proprietary driver — the app window paints BLACK and stderr fills with
+// "Failed to create GBM buffer of size WxH: Invalid argument" (usually with an
+// "X11 Error: GLXBadWindow" alongside). Disabling that renderer path is the
+// standard upstream workaround.
+//
+// It has to be in the environment BEFORE the app process starts: WebKit reads
+// it when it forks its web process, and Bun's `process.env` writes do NOT
+// propagate to children (verified — a child sees the variable unset), so the
+// app cannot set this for itself from inside src/bun/. Every launch path has to
+// inject it: the dev scripts here, and the wrapper script that release.ts ships
+// beside the packaged launcher.
+//
+// Scoped to NVIDIA so Intel/AMD keep hardware compositing, and an explicit
+// value from the user's environment always wins.
+import { existsSync } from "node:fs";
+
+export function needsDmabufWorkaround(): boolean {
+	return (
+		process.platform === "linux" &&
+		process.env.WEBKIT_DISABLE_DMABUF_RENDERER === undefined &&
+		existsSync("/proc/driver/nvidia")
+	);
+}
+
+/** `process.env` plus the workaround, ready to hand to Bun.spawn. */
+export function launchEnv(extra: Record<string, string> = {}): Record<string, string | undefined> {
+	return {
+		...process.env,
+		...(needsDmabufWorkaround() ? { WEBKIT_DISABLE_DMABUF_RENDERER: "1" } : {}),
+		...extra,
+	};
+}