refactor(js): extract crypto module from app.js monofile (§3.1 start)

First slice of the app.js split. Approach: ordered classic-script files
loaded via separate <script> tags (argon2.js → app.crypto.js → app.js),
NOT ES modules / a bundler. Classic scripts share one global lexical
environment, so consts/functions cross-reference across files exactly as
in the monofile — zero call-site rewrites, near-zero risk. Chosen over the
audit's esbuild/ES-module suggestion because the code is written entirely
in global scope (functions call each other by bare name everywhere).

- js/app.crypto.js: KDF (PBKDF2 + Argon2id), verifier, AES-GCM encrypt/
  decrypt, key persist/restore. Verified byte-for-byte identical to the
  original block before removal; no duplicate const across the two scripts.
- index.html + BuildAssets whitelist + test harness updated for the load
  order. Harness CONCATENATES app.crypto.js + app.js (node:vm doesn't share
  top-level const across separate runInContext calls the way browsers share
  it across <script> tags); argon2.js stays a separate IIFE.
- Runtime-validated: rebuilt exe unlocks via quick-unlock and loads/decrypts
  entries — the extracted crypto (restoreCryptoKey, verifierFromKeyHex,
  decryptPwd) works from the separate file. 42/42 tests green.
- Docs: CLAUDE.md "Découpage frontend" (pattern + rules), file map, tests
  README, CODE_AUDIT §3.1.

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

index.html

@@ -1193,6 +1193,7 @@
 </div>
 
 <script src="js/argon2.js"></script>
+<script src="js/app.crypto.js"></script>
 <script src="js/app.js"></script>
 </body>
 </html>