Rename exported save file from IdleForge to Depth Engine

[keepithandy]

Goal

Clean up the remaining exported save filename so the repo identity consistently says Depth Engine instead of the old IdleForge name.

Current behavior

exportSave() currently downloads saves as idleforge-save.json.

Proposed work

• Rename the exported save filename to something like depth-engine-save.json.
• Keep the export format unchanged.
• Keep import compatibility unchanged.
• Do not change the save key unless a separate migration issue is opened.

Acceptance checks

• Clicking Export Save downloads a file with a Depth Engine filename.
• Existing exported JSON still imports correctly.
• node smoke_depth_engine_core.mjs still passes.
• node smoke_rat_cellar_content.mjs still passes.

Guardrails

This is a repo identity cleanup only. Do not introduce a save migration or change the browser storage key in this issue.