fix(search): use canonical severity labels

[vsolano9]

Closes #30

What changed

• remove the private long-form severity label map from SearchResults
• use the canonical shared labels and SeverityLevel type for the filter range
• narrow adjacent range values with the existing severity guard rather than using any
• preserve the deliberate short labels used by dense listings

Validation

• Node 22.23.2: npx vitest run — 14 files, 121 tests passed; full suite repeated 20 times
• Node 22.23.2: npx tsc --noEmit
• Node 22.23.2: npm run lint — no warnings or errors
• Node 22.23.2: npx prettier --check .
• Node 22.23.2: production npm run build with the CI placeholder environment
• headed browser: desktop and 390×844 production mobile visual inspection
• headed browser: all five canonical accessible names; range transitions 1–5 → 2–5 → 2–4 → 3 → reset; zero browser errors/warnings

No dependency or lockfile changes.