Sparkline mixed-NA test does not assert absence of NApx in output

**Source:** PR #278 review (round 1)
**Severity:** MEDIUM
**File:** `tests/testthat/test-cost-tracking.R:290-299`

The mixed-NA test confirms non-null and correct class but does not verify the rendered HTML is free of invalid CSS. The NA bar height bug produces `"NApx"` which this test misses.

**Suggested fix:** Add `expect_false(grepl("NApx", as.character(result)))` to the test.