Remove CEF: the web content crate, ElyMark's page, their packaging, licences and tests (C14.5)

PRD and agents.md name the native route; ElyMail's body will not use
CEF or Electron. ElyMark's package carries the KaTeX fonts' OFL.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.cargo/config.toml

@@ -1,6 +1,2 @@
 [target.'cfg(all(windows, target_env = "msvc"))']
 rustflags = ["-C", "target-feature=+crt-static"]
-
-[env]
-# cef-dll-sys builds against the CEF that scripts/fetch-cef.sh fetched.
-CEF_PATH = { value = "engines/cef", relative = true }