[Feature]: refine Quick Start composer interactions #672

Problem

Quick Start currently presents template upload, art style, direction selection, sending, and interruption as inconsistent composer controls. The entry textarea also treats Enter as a newline, which slows the primary send flow.

Proposed outcome

Unify the Quick Start entry and in-progress composers around a compact bottom-aligned interaction model. Keep configuration controls visible before generation, provide a real stop action while planning or starting, and make the keyboard behavior match a message composer.

Scope

• Keep the Quick Start composer at the bottom in entry and in-progress states.
• Place template upload and art-style controls on the left, with the selected style and generation direction on the right.
• Use compact custom icons and a solid animated direction slider without shader decoration.
• Reuse the composer action button for send and interruption.
• Cancel pending Agent planning or uploaded-template startup and restore the pending prompt.
• Submit with Enter; preserve Command+Enter for a textarea newline; do not submit during IME composition.
• Add focused regression coverage for the visible controls, interruption, and keyboard behavior.

Out of scope

• Editing art style or generation direction after project creation.
• Changing backend Project or generation contracts.
• Shader or Paper Shaders integration.

Acceptance criteria

• The entry composer remains compact and bottom-aligned with controls above the textarea.
• The selected art style is visible and persists with the Quick Start draft.
• Direction selection uses a smooth, opaque slider popover and reaches single-, four-, and eight-way values.
• Send and stop actions remain inside the textarea surface.
• Stopping pending work returns the composer to an editable idle state without dispatching generation.
• Enter submits once, Command+Enter remains available for newline insertion, and composing Enter is ignored.
• The active-run composer uses the same bottom spacing, corner radius, and circular stop action.

Dependency

Depends on #603 for the controlled art-style field and Project UI contract.