feat: Always render single/multi selection options vertically (M2-10899)

[hotavocado]

Description

🔗 Jira Ticket M2-10889

Single and multiple selection items now render their options in one vertical column at every breakpoint, replacing the two-column split that applied at ≥600px.

This is the layout the app already used below 600px, so it is an existing, validated design now applied consistently across all widths.

Per the discussion on M2-10899, this assumes no configuration flag. Rather than gate the old and new layouts per item, we are dropping the multi-column approach entirely and always going vertical, to keep behavior consistent across breakpoints and platforms. Existing items pick up the new layout automatically — there is no per-item setting, no backend field, and no admin panel change.

Portrait (tile style) responses are unaffected: PortraitGrid keeps its current wrapping grid.

Changes

• RadioItem/RegularGrid.tsx and CheckboxItem/RegularGrid.tsx render a single flex column at gap="16px". Option cards span the parent's full width via the default align-items: stretch, so no explicit width rule was needed.
• Removed splitList (src/shared/utils/helpers/splitList.ts) and its barrel export. It was the two-column helper and had no other callers or tests.
• Removed the now-unused useCustomMediaQuery imports from both grids — lessThanSM was their only consumer.

Padding, gap, and card styling are unchanged. Option ordering, randomizeOptions, hidden-option filtering, tooltips, images, and palette colors are all untouched.

Net: 41 insertions, 115 deletions.

Testing

• yarn tsc --noEmit — 0 errors
• yarn lint:check — 0 errors, 32 warnings (unchanged from dev; none introduced by this change)
• yarn test — 751 passed / 44 files
• Manual verification against a real applet: options should render as a single full-width column, identical above and below the 600px breakpoint. Pending design sign-off against the Figma frame.

Checklist

Functionality

• The feature behaves correctly in practice and fulfills the intended business purpose
• The implementation accounts for edge cases, avoids subtle logical errors, and handles somewhat rare failure states (e.g. offline mode for mobile, 3rd party being down, etc)

Testing

• Verify there are automated tests added that meaningfully cover critical behavior and failure cases
• Code coverage does not go down as result of this change
• Test suite passes

Security & Data Privacy

• Verify there is no chance we would accidentally log PII to application logs
• Verify this addition does not materially affect our security attack surface, and if so it has undergone security review
• All inputs are sanitized
• New dependencies are well maintained and have significant justification for being added to the project

Logging/Monitoring

• Logging is implemented for this change such that you could troubleshoot this feature in production
• The change/feature is able to be monitored in production

Performance

• This change does not introduce n+1 queries or other performance issues within our expected scale (e.g. missing indexes on frequently queried columns, frequently updating tables that are accessed often)

Readability

• All commented out code is removed
• Debugging code including extraneous log lines are removed
• Code is easy to understand through naming and structure; comments explain intent or non‑obvious decisions

Change Safety

• Backend changes are backwards compatible with old clients, or it is well known they are not and a deployment/rollout plan is in place. This include backend changes being compatible with old mobile app versions, as well as applet versioning within Curious.
• Destructive database migrations are rolled out in stages. For example, renaming a column means adding a new column and migrating the existing data to that columns in one deployment. Then monitoring to ensure that field isn’t used, and finally removing that old column in a separate deployment.

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[divbzero]

Do not merge yet. Wait for QA confirmation.

[divbzero]

Also wait to add an admin banner / notify admin users.