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>

docs/gotchas.md

@@ -67,9 +67,25 @@ touching the adjacent code.
   `libasar.so => not found` line in that output is a red herring (it ships beside
   the wrapper and resolves at load). Install list per distro is in
   [platform-notes.md](platform-notes.md).
-- Benign startup noise on Linux — all non-fatal, the app runs normally:
-  `X11 Error: GLXBadWindow (code 170)`, `Failed to create GBM buffer of size
-  WxH: Invalid argument` (compositor/GLX chatter from the GTK webview), and
+- **Black window on NVIDIA + `Failed to create GBM buffer of size WxH: Invalid
+  argument`** (usually with `X11 Error: GLXBadWindow (code 170)`). The app boots,
+  logs "miniSCP started", reports no error — and paints nothing. WebKitGTK's
+  DMABUF renderer can't allocate buffers on the NVIDIA proprietary driver;
+  `WEBKIT_DISABLE_DMABUF_RENDERER=1` is the upstream workaround.
+  - **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). Setting it in
+    `src/bun/index.ts` — even in a module imported before `electrobun/bun` —
+    does nothing. It must be in the environment before the app process starts.
+  - Hence every launch path injects it: `scripts/linux-gpu.ts` (`launchEnv()`)
+    for `bun run dev` / `bun run start`, and the `miniSCP.sh` wrapper that
+    `release.ts` writes beside `bin/launcher` in the tarball. **A new launch
+    path must call `launchEnv()` or it will render black on NVIDIA.**
+  - Gated on `/proc/driver/nvidia` so Intel/AMD keep hardware compositing, and
+    an explicit `WEBKIT_DISABLE_DMABUF_RENDERER` from the user always wins.
+  - A "does it boot?" log check does NOT catch this — grep for the GBM line, or
+    look at the window.
+- Benign startup noise on Linux — non-fatal, the app runs normally:
   `libayatana-appindicator is deprecated. Please use libayatana-appindicator-glib`
   (upstream electrobun's choice, not ours).
 - **Window-open flash prevention is three-layer** — remove any one and the