refactor(js): extract vault-health module from app.js monofile (§3.1)

Seventh slice of the app.js split. Moves the vault-health dashboard
(computeHealthCache, healthScoreBand, renderHealthDashboard/Section,
openEntryForFix, entryAgeDays + scoring consts) to js/app.health.js. Pure
declarations, no top-level side effects → loads before app.js. Uses
computeStrength/decryptPwd/state/api via shared global scope at call time.

- Byte-for-byte identical extraction; syntax OK on all eight app parts.
- auditCache/auditFilter sit in this var block but drive the separate
  Audit-log viewer in app.js — they ride along and resolve cross-file via
  shared scope (documented).
- index.html + BuildAssets whitelist + harness APP_PARTS updated.
- 62/62 tests green.

app.js: 11936 → 9545 lines (7 modules extracted).

NOTE: quick search is NOT contiguous (interleaved with cheatsheet +
history-modal code, lines 886-1063), so a clean byte-identical extraction
isn't trivial — deferred.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CLAUDE.md

@@ -37,6 +37,7 @@ js/app.totp.js    (TOTP RFC 6238 + TOTP/custom-field crypto — extrait §3.1)
 js/app.favicon.js (favicon fetch/cache + faviconHost — extrait §3.1)
 js/app.import.js  (export container + import CSV/JSON — extrait §3.1)
 js/app.backup.js  (auto-backup planifié — extrait §3.1)
+js/app.health.js  (vault health dashboard — extrait §3.1)
 js/app.js         (le reste : state, Bridge, api, UI…)
 js/app.sync.js    (WebDAV + merge — extrait §3.1, APRÈS app.js car
                    effet de bord top-level `Bridge.onWebdavResult = …`)
@@ -107,6 +108,7 @@ seule `api()` est stubbée).
 | Favicons (fetch/cache, `faviconHost`) — extrait §3.1 | `js/app.favicon.js` |
 | Import/export frontend (CSV/JSON parse, export container) — extrait §3.1 | `js/app.import.js` |
 | Auto-backup frontend (planifié, chiffré) — extrait §3.1 | `js/app.backup.js` |
+| Vault health dashboard — extrait §3.1 | `js/app.health.js` |
 | Sync frontend (WebDAV, snapshot, merge) — extrait §3.1 | `js/app.sync.js` |
 | Argon2id vendé (bundle `@noble/hashes`, IIFE) | `js/argon2.js` |
 | HTML racine | `index.html` |