Align Rollup config with package-wide ESM mode

[Copilot]

This addresses the review thread on module semantics by making the tooling consistent with the repository’s package-level ESM configuration. Instead of scoping ESM to a single config extension, the Rollup config now participates in the app’s module mode directly.

• What changed

  • Renamed rollup.config.mjs → rollup.config.js
  • Updated Makefile build target dependencies and invocation to reference rollup.config.js

• Why this matters

  • Removes mixed signaling between package-level "type": "module" and tool-specific file extensions
  • Keeps bundling config aligned with the stated module mode across the app/tooling boundary

• Config/reference update

  $(STATIC_DIR)/main.js: $(shell find ts -name "*.ts") rollup.config.js tsconfig.json
  	npx rollup --config rollup.config.js