Encrypt lockbox content in SQLite database to prevent plaintext access

[BradGrensewich]

This PR encrypts content field when it is stored before storing to SQLite DB and decrypts it when loading from.

Goal
-prevent trivial inspection of lockbox contents before timer has expire
-no other changes to app

[bleriotnoguia]

Lockbox Local v2.0 is available ! See https://github.com/bleriotnoguia/lockbox-local

This v2 is a complete rewrite using modern technologies (Tauri 2.0 + React 19 + Rust), bringing AES-256-GCM encryption, a master password, categories, search, theming, and a much smaller footprint see comparison table below.

Comparison with v2

Feature	v1 (Java)	v2 (Tauri)
Bundle size	~100 MB	~15 MB
RAM	~150 MB	~50 MB
Encryption	No	AES-256-GCM
Master password	No	Yes
Categories	No	Yes
Free tags	No	Yes
Search	No	Yes
Dark theme	No	Yes
Cancel countdown	No	Yes
Extend delay	No	Yes
Reflection modal	No	Yes
Penalty mode	No	Yes
Panic code	No	Yes
Scheduled unlock	No	Yes
Access log	No	Yes
Self-control stats	No	Yes
Export format	.lbf (text)	.json (HMAC-signed)
EN/FR localisation	No	Yes

All V2 releases are available on the Releases page.