fix(gui): keep composer controls within narrow session layout (issue 208)

[boazikum]

this pr addresses issue #208 Control button UI overflow by doing the following:
When the side panel is open, the available width for the chat composer becomes smaller. The model picker was allowed to hold onto too much width, so the “Stop” button could be pushed outside the composer.
The fix does three small things:

1. Allows the composer’s control row to shrink.
2. Allows the model dropdown to shrink from its usual maximum width and truncate the model name with ….
3. Marks the “Stop” button as non-shrinking, so it stays fully visible and clickable.
   So at narrow widths, the model label becomes shorter rather than the action button overflowing.