fix(app): scroll overflowing composer controls

[iamdavidhill]

Issue for this PR

No linked issue; fixes composer toolbar overflow reported during desktop testing.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Makes the agent/model/variant controls horizontally scrollable while keeping the plus button and Submit/Queue/Steer actions fixed. Gradient masks appear only at edges with hidden content and disappear at the scroll endpoints, without adding padding. Keeps a 4px gap beside the plus and a 12px gap beside the submit actions, mirrored in RTL.

Adds a long-label Storybook fixture and browser coverage for scrolling, endpoint spacing, keyboard/menu access, resizing, and optional Queue/Steer actions.

How did you verify your code works?

• bun typecheck in packages/app passed.
• Repository pre-push typechecks passed (33 tasks).
• bun run test:components composer.spec.ts --workers=2 --grep-invert 'raises the docked composer' in packages/app: 14 passed.
• The existing dark-theme background assertion fails when run with the full composer suite and was excluded from the final targeted run. Dark mode was also checked visually through Storybook.

Screenshots / recordings

Checked the production composer in Storybook in light/dark mode, narrow layouts, and RTL. Screenshots are not attached; the interactive reproduction is OpenCode → Composer → Flow → Toolbar Overflow, with controls for no shortcut, Queue, and Steer.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR