Mechanical fixes across 13 files:
- isNaN() → Number.isNaN() (all values already numeric from parseFloat/parseInt)
- let → const where never reassigned
- Math.pow() → ** operator
- Unnecessary continue in for loop
- Useless string escape in test description
- Missing parseInt radix parameter
- Remove unused private class member (write-only counter)
- Prefix unused function parameter with _
Config: suppress noImportantStyles (CSS !important is intentional) and
useLiteralKeys (bracket notation used for computed/dynamic keys) in
biome.json. Remaining 49 warnings are all noExcessiveCognitiveComplexity
(already configured as warn, safe to address incrementally).
