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>

js/tests/harness.js

@@ -25,7 +25,12 @@ const path = require('node:path');
 const vm = require('node:vm');
 const { webcrypto } = require('node:crypto');
 
-const APP_JS = path.join(__dirname, '..', 'app.js');
+// The frontend is split into ordered classic-script files (§3.1). In the
+// browser they share one global lexical environment; node:vm does NOT share
+// top-level const/let across separate runInContext calls, so we CONCATENATE
+// the app.* parts (in <script> load order) into one script. argon2.js is a
+// self-contained IIFE and loads separately (see below).
+const APP_PARTS = ['app.crypto.js', 'app.js'].map(f => path.join(__dirname, '..', f));
 
 // In-memory Storage stub (Web Storage API surface used by app.js).
 function makeStorage() {
@@ -119,7 +124,10 @@ function loadApp(overrides = {}) {
     const ARGON2_JS = path.join(__dirname, '..', 'argon2.js');
     vm.runInContext(fs.readFileSync(ARGON2_JS, 'utf8'), sandbox, { filename: 'argon2.js' });
 
-    let src = fs.readFileSync(APP_JS, 'utf8');
+    // Concatenate the app.* parts in load order (see APP_PARTS). Newline
+    // separators keep line-based errors legible; shared global scope is
+    // preserved because it's a single script run.
+    let src = APP_PARTS.map(p => fs.readFileSync(p, 'utf8')).join('\n;\n');
 
     // Export epilogue — surface the lexical (const) symbols we test, plus a
     // couple of function-decl seams for convenience. Kept in one place so the