[feat] Notification content visibility privacy options for locked and unlocked states

Current Behavior:
Notifications expose full content (Sender + subject + message) even when the mail client is locked using Applock.

Proposed Solution:
Introduce granular notification settings configured independently for the application's locked and unlocked states. The unlocked state must apply to both scenarios: when Applock is configured but currently unlocked, and when Applock is not configured at all.

Requested Options (configurable per state):

Sender + subject + message

Sender + subject (Suggested default for unlocked state)

Sender only

No sender and subject (Suggested default for locked state)

Use Case:
This prevents data exposure when the application is locked and enables strict privacy preferences globally, regardless of the overall Applock configuration.

No files, tests, or entry points are named. Start by locating notification generation and the Applock state handling, then trace how notification preferences are configured. Done means locked and unlocked states each support all four requested visibility levels, with the stated defaults and unlocked behavior applying whether Applock is configured or not.

Written by the indexing model from the issue text.