Burn down ESLint and Angular lint debt

[fpigeonjr]

Parent

#562

What to build

Burn down the TypeScript, Angular, and template-accessibility lint debt exposed by the ESLint migration in #566. The migration currently reports 1,632 warnings in the root workspace and 5 in test-app. Coordinate independently grabbable cleanup slices, prevent the baseline from regressing, and progressively promote resolved rules from warnings to errors.

This debt cleanup is tracked separately from the Angular 19→21 serial migration so it remains visible without blocking unrelated upgrade steps.

Acceptance criteria

• CI prevents the accepted lint baseline from increasing
• Existing ESLint findings are resolved or covered by an explicit, documented rule decision
• Resolved rule categories are promoted from warnings to errors
• Root and test-app lint complete with no warnings or errors outside documented exceptions
• Existing automated tests remain green throughout the cleanup

Blocked by

• tslint → ESLint + Angular template a11y gate (ADR-0006) #566