[Feat] No Tasks Left Message

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**Is your feature request related to a problem? Please describe.**
When the task list is empty, the screen appears blank, which can confuse users and make the app feel incomplete. There is no indication that the task list is intentionally empty or a suggestion to add new tasks.

**Describe the solution you'd like**
Display a "No tasks yet" message when the task list is empty.
Include an illustration or icon to make the message visually appealing.
Add a call-to-action button below the message, such as "Add Task", to encourage users to create their first task.
The message should adapt to the current theme (light or dark mode).

**Additional context**
Positioning: Center the message and icon vertically on the screen.
Animations: Add a subtle fade-in animation when the message appears.

Start by locating the Flutter screen that renders the task list and identify how its empty state is handled. Add the centered empty-state message, illustration or icon, Add Task action, theme adaptation, and fade-in behavior described in the issue; verify the result in both light and dark modes.

Written by the indexing model from the issue text.