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>

index.html

@@ -1197,6 +1197,7 @@
 <script src="js/app.favicon.js"></script>
 <script src="js/app.import.js"></script>
 <script src="js/app.backup.js"></script>
+<script src="js/app.health.js"></script>
 <script src="js/app.js"></script>
 <script src="js/app.sync.js"></script>
 </body>