edge: don't take the host process down when the WebView2 controller/environment can't be created (session 0)

[peterpla]

What

On a non-visible window station — a process in Windows session 0 with no
interactive desktop (a service, an S4U scheduled task, a boot task) — WebView2
controller creation completes with a success HRESULT but a nil controller.
CreateCoreWebView2ControllerCompleted guards only the HRESULT, then dereferences
the controller, so the host process panics on a nil pointer inside webview.New.
EnvironmentCompleted has the same shape via log.Fatalf, which also exits the
process. Neither lets the caller fall back to a non-GUI path.

Fixes #86.

Change

pkg/edge/chromium.go — wire the async failures into the nil-return contract
New already has (New → nil when CreateWithOptions → Embed returns false),
with no public signature change:

• Test the HRESULT with int32(res) < 0 (HRESULT is signed 32-bit) and guard
  controller/env for nil in the two completion callbacks; on failure, record an
  embedErr and set inited to unblock Embed's message pump instead of
  log.Fatalf/dereferencing nil.
• Embed returns false when embedErr is set (matching its existing
  log.Printf(...); return false for other setup failures), so New returns nil.

Note on the width: the observed failure HRESULT is 0x8000401a, which is negative
as int32 but not as int64 — so the original int64(res) < 0 never fired for
a real failure code. Narrowing to int32 makes the HRESULT branch work; the nil
guard additionally covers a nil out-pointer returned alongside a success HRESULT.

Verification

Standalone program calling only webview.New(false), launched on a session-0
window station (over PowerShell Direct), Windows 10 + WebView2 Evergreen
151.0.4129.86:

• Before: panic (nil pointer dereference) at chromium.go:190, process exit 2.
• After: webview.New returns nil, process exit 0.

Both runs confirmed a non-visible (session-0) window station.

Scope

A smaller change is possible if preferred: the controller nil-guard alone fixes the
crash, and the log.Fatalf → error-return part can be dropped.

[jchv]

Woah, I don't think I ever expected anyone to try go-webview2 in Session 0. I don't really have a policy for LLM patches yet but this looks trivial enough that it is probably not worth worrying about. I should probably run this locally before merging it but it does LGTM.

[peterpla]

Headless browser-based webcam image captures on a schedule (e.g., sunrise to sunset, every 5 minutes), auto-start as session-0 lets captures continue before the user logs in. Not an obvious scenario :) Thanks for your work on go-webview2!