fix: make implicit append else branch explicit with console.warn

[JuliaKalder]

Summary

Changed the implicit else at template-insert.js:325-328 to an explicit else if (mode === "append") branch with a console.warn for unknown insertMode values. This makes the code self-documenting and prevents silent fallback behavior for any future new insertMode values.

Changes

• modules/template-insert.js: Made the append-mode else branch explicit with console.warn fallback for unknown modes

Testing

• Verified JavaScript syntax with node --check
• Code follows existing patterns in the same function

Fixes #166