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/platform-notes.md

@@ -96,18 +96,26 @@ either way — the tray is a convenience, and every action it offers exists in-a
   already macOS-only, and the panel context menus are now HTML on every platform,
   so nothing is missing at the UI level — but do not add a feature that depends
   on a native menu.
-- **Benign startup noise**: `X11 Error: GLXBadWindow (code 170)`, `Failed to
-  create GBM buffer of size WxH: Invalid argument`, and a deprecation warning
-  pointing at `libayatana-appindicator-glib`. None are fatal.
+- **NVIDIA renders black without the DMABUF workaround.** WebKitGTK can't
+  allocate GBM buffers on the proprietary driver, so the window paints nothing
+  while the log looks perfectly healthy. Every launch path sets
+  `WEBKIT_DISABLE_DMABUF_RENDERER=1` when `/proc/driver/nvidia` exists —
+  `scripts/linux-gpu.ts` for dev, the `miniSCP.sh` wrapper in the tarball for
+  releases. The app can't do it itself (Bun's `process.env` writes don't reach
+  child processes); see [gotchas.md](gotchas.md#linux).
+- **Benign startup noise**: a deprecation warning pointing at
+  `libayatana-appindicator-glib`. Not fatal.
 - **Unsigned, unpackaged**: the release artifact is a portable tarball — no
   AppImage, `.deb`, `.desktop` entry or icon installation. Unpack and run
-  `bin/launcher`.
+  `miniSCP.sh` (which execs `bin/launcher`).
 
 ## First-Linux-boot smoke test
 
 1. Install the AppIndicator package for your distro (table above), then
-   `bun run start`. The window opens and the tray icon appears (or doesn't, on
-   bare GNOME — that's the extension caveat, not a failure).
+   `bun run start`. The window opens **and actually renders the UI** (a black
+   window on NVIDIA means the DMABUF workaround didn't apply — check for
+   `Failed to create GBM buffer` in the log), and the tray icon appears (or
+   doesn't, on bare GNOME — that's the extension caveat, not a failure).
 2. Right-click a file row: menu appears at the pointer, Send to Other Panel is
    first, Permissions is present, Escape / click-outside / arrow keys + Enter all
    work, and a right-click near the bottom-right flips the menu instead of
@@ -122,7 +130,7 @@ either way — the tray is a convenience, and every action it offers exists in-a
 6. Move to Trash lands the item in the desktop's trash; connections.json is 0600.
 7. `bun scripts/release.ts --target linux-x64` produces
    `release/linux-x64/miniSCP-Build<N>-linux-x64.tar.gz`; unpack it elsewhere and
-   run `bin/launcher`.
+   run `miniSCP.sh`.
 
 ## Windows known issues (upstream electrobun, documented not fixed)