This commit introduces the `Sentiment` enum and updates the main UI components:
- Adds a `Sentiment` enum with friendly names and leading icons for `HAPPY`, `NEUTRAL`, and `SAD`.
- Replaces the previous `Greeting` class with this new `Sentiment` based approach.
- Updates the main UI (`App`) to use `FilterChip` for sentiment selection.
- Adds a `TextField` for user comments and a "Submit" button.
