EDU-19729: Add follow-up panel to FeedbackSection

[mariana-caetano]

Description

Adds an inline follow-up panel to FeedbackSection that opens automatically right after the user votes ("Was this helpful?" -> Yes/No), letting them optionally leave a comment and/or email for more context.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Requires change to documentation, which has been updated accordingly.

How it works

• The vote is saved immediately, exactly as before (POST /api/feedback/), so existing tracking is unaffected.
• Right after the vote, the panel opens automatically (panelStage: 'open') with a textarea ("Tell us more") and an optional email field.
• Send submits the detailed feedback as a second, independent row to the same spreadsheet already used for votes (POST /api/feedback/).
• Skip closes the panel without sending anything.
• New props: collectDetailedFeedback (default true), detailedFeedbackEndpoint, sendDetailedFeedback, defaultPanelStage - all overridable, keeping the component backward-compatible for consumers that don't want the follow-up panel.

How to test

1. Open this repository locally.
2. Checkout the EDU-19729 branch.
3. Open a terminal and run npm run storybook.
4. Open the Storybook server and, in the left menu, find the FeedbackSection component.
5. In the FeedbackSection menu, check the new stories: FollowUpOpen, FollowUpSubmitted, FollowUpError and FollowUpOpenSmall.
6. On any story, click Yes or No to see the follow-up panel open automatically, and then try Send and Skip.

P.S.: None of the actions in the Storybook environment persist data because there's no real backend behind /api/feedback/. Voting (Yes/No) will still visually open the follow-up panel (the request silently fails without showing an error), but clicking Send afterward will land in the error state.

[github-actions]

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[karenkrieger]

LGTM