Version: 1.6.0

fix: multiple bug fixes and rebranding to SecureDrop

- fix(note): cache ciphertext after first fetch to prevent note deletion on wrong password
- fix(note): show 'Wrong password' error with 5s auto-hide and retry button
- fix(upload): add missing metaStep, encryptBtn, backBtn elements to upload.html
- fix(viewFile): replace non-existent DOM IDs (metaFileName, fileMeta) with existing fileName element
- fix(viewFile): remove file preview (image/PDF/text) — download-only UI
- refactor(layout): split layout() into renderHead/renderStyles/renderScripts helpers to stay within line limits
- chore: rebrand all occurrences of SecureSource → SecureDrop across HTML, templates, package.json and README