fix: harden crash-prone code paths

[everpcpc]

This PR focuses on reducing hard-crash surfaces reported by new users.

Changes:

• SwiftData: remove duplicate CustomFont entry from Schema and improve ModelContainer init error message (still fails fast on init failure).
• Storage: avoid Dictionary(uniqueKeysWithValues:) traps by using uniquingKeysWith: in upsert paths.
• Logging: harden LogStore when SQLite open fails and when sqlite3_column_text returns NULL.
• Quick Actions: defer handling until DatabaseOperator.shared is available (no db!).
• Reader: avoid as! traps when dequeuing/making Webtoon cells.
• Widgets/Settings: remove URL(string:)! force unwraps for links/deep links.
• Upscaler: add input/output array bounds guards to prevent out-of-bounds access.

Notes:

• No functional changes intended beyond crash hardening; when SwiftData cannot initialize, the app still terminates early (expected).