The #3018 carry-forward reassigns msgs but it was declared const, throwing a
TypeError that surfaced as 'Failed to load conversation messages' on every mobile
message (v0.51.161-166). Change to let.
Adds a static JS runtime-error lint guard (eslint.runtime-guard.config.mjs +
tests/test_static_js_runtime_lint.py) using no-const-assign/no-import-assign — the
exact class node --check and source-presence tests miss. Dev-only dependency; app
stays pure Python + vanilla JS.
nesquena-hermes