fix: distinguish unassessed job scores

[sangowu]

What changed

• render unassessed null/undefined scores as a gray em dash
• preserve numeric zero as a real red 0 score
• add a regression test and update the changelog and bug-fix log

Root cause

scoreBadge() converted the API's score: null with Number(null), which JavaScript evaluates as 0. This made jobs without a current-CV match_score look like genuine zero-score matches.

Validation

• 248 passed
• Ruff passed
• Python compileall passed
• frontend syntax check passed
• direct Node behavior check confirmed null and zero badges remain distinct
• live / response contains the corrected frontend logic