[audit] Move shared context-menu image resolution out of the WebKit adapter

[bnema]

Summary

CEF currently depends on a resolver constructed from the WebKit adapter package for shared context-menu image handling.

Why this matters

This couples the primary CEF path to a WebKit-specific package boundary and makes future engine cleanup harder than necessary.

Evidence

• internal/bootstrap/engine.go:118 wires CEF with webkit.NewContextMenuResolver().
• internal/infrastructure/webkit/context_menu.go:26 describes that resolver as a WebKit-specific seam.

Suggested scope

• Extract the HTTP/data-URI image resolver into an engine-neutral package.
• Depend on the shared abstraction from both CEF and WebKit.
• Keep behavior unchanged while reducing package coupling.

Acceptance criteria

• CEF no longer imports or depends on a WebKit-specific resolver constructor.
• Shared image resolution behavior remains covered by tests.
• Package ownership reflects shared infrastructure rather than backend-specific code.

Audit context

• Audit date: 2026-06-10
• Audited at commit: 5eba8fd5