refactor(js): extract import/export module from app.js monofile (§3.1)

Third slice of the app.js split. Moves the encrypted export container,
CSV/JSON import parsing (parseCSV, findColumn, parseEntriesFromCSV,
parseEntriesFromJSON), and the doImport/doExport/doExportCSV flows to
js/app.import.js. encryptImportEntry moves here too (also called by
app.sync.js — resolved via shared global scope at call time).

- Byte-for-byte identical to the extracted block; no duplicate const;
  no top-level import ref left in app.js.
- Load order: BEFORE app.js (pure declarations, no top-level side effects),
  alongside app.crypto.js. Full order: argon2 → crypto → import → app → sync.
- index.html + BuildAssets whitelist + harness APP_PARTS updated.
- Safety net: the 14 CSV tests exercise parseCSV/parseEntriesFromCSV from
  the extracted file and stay green (42/42).

app.js: 11936 → 10253 lines (crypto + sync + import now separate, ~1700
lines moved into 3 modules).

NOTE: assets.res not regenerated here (needs brcc32/Delphi) — run
BuildAssets before the next Delphi build to embed js/app.import.js.

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

index.html

@@ -1194,6 +1194,7 @@
 
 <script src="js/argon2.js"></script>
 <script src="js/app.crypto.js"></script>
+<script src="js/app.import.js"></script>
 <script src="js/app.js"></script>
 <script src="js/app.sync.js"></script>
 </body>