Convert the Electron shell to strict TypeScript #22

Outcome

The Electron main process, preload bridge, and application menu are strictly typed across process boundaries.

Stack position

6 of 7 — stacked on the typed browser support layer.

Acceptance criteria

• Electron main, preload, and menu modules are converted.
• IPC and preload interfaces use shared contracts and expose no accidental capabilities.
• Window lifecycle, background-service, focus, and packaging behavior remain unchanged.
• Corresponding tests are converted in the same PR.
• No runtime TypeScript loader or module-system redesign is introduced.
• The temporary JavaScript boundary shrinks and all checks pass.
• A self-contained ELI5 HTML explanation of the Electron boundary is committed under doc/explanations/ and linked from the PR.

ELI5 dependency

Generate the explanation with the forthcoming eli5html skill once it becomes available. The TypeScript implementation may proceed independently, but this issue and its PR are not complete until the skill-produced document is present and verified.