[HOLD on #100734] [LegendList 7/7] feat: migrate remaining FlatLists and restrict legacy imports

[chrispader]

Several screens and native drag-and-drop still use FlatList after the FlashList migration. This PR moves the remaining application lists to direct LegendList package imports and adds lint restrictions to prevent new FlatList or FlashList usage.

@roryabraham @dmkt9

Explanation of Change

Layer 7 of the seven-part LegendList and chat pagination stack. Consumers import {LegendList} from @legendapp/list/react-native and {AnimatedLegendList} from @legendapp/list/reanimated, with their types coming from the package exports. This layer covers reactions, autocomplete, static search, filter chips, trip previews, training and attachment carousels, receipt previews, and the native draggable list. It removes unused list components and the FlatList-backed drag dependency and patches while retaining the keyboard context and render-queue helper still referenced elsewhere. Regression tests cover restricted imports, carousel state and paging, and native drag activation, cancellation, and autoscroll.

This draft depends on layer 6 and targets upstream main, so GitHub's Files changed tab includes earlier layers until they merge. Review only the focused layer 7 diff. These are ordinary upstream PRs because GitHub native stacks do not support cross-fork branches; their branches are managed locally with gh stack. Rebase this branch after its dependencies land.

Review order: 1. Foundation → 2. Message edit scrolling → 3. Loading skeleton consistency → 4. Report actions → 5. Chat pagination loading → 6. Remaining FlashList consumers → 7. Remaining FlatList consumers and restrictions. Merge and rebase in this order.

Fixed Issues

$ #98994
$ #99202

This draft is part of the migration split from #98270. No approved proposal is linked.

Tests

Manual platform checks have not been run; the PR remains a draft.

Before marking ready:

1. Exercise reactions, autocomplete, static search, and horizontal filters. Verify keyboard and selection behavior.
2. Page through training and attachment carousels with arrows and gestures. Resize the window and verify the active attachment remains aligned.
3. Capture multiple receipts and rotate the device. Verify the preview list orientation, sizing, and end scrolling.
4. Reorder distance waypoints on Android and iOS. Verify long-press and handle activation, disabled rows, cancellation, holding a row at either edge, and footer/add-row behavior.

• Verify that no errors appear in the JS console

Offline tests

Not yet tested manually. Reorder cached waypoints and inspect queued receipt previews offline, then reconnect and verify the saved order and pending receipts.

QA Steps

Repeat the manual steps above on native Android/iOS and supported web platforms. Native drag-and-drop needs device verification before this draft is ready.

• Verify that no errors appear in the JS console

PR Author Checklist

• I linked the correct issue in the ### Fixed Issues section above
• I wrote clear testing steps that cover the changes made in this PR
  • I added steps for local testing in the Tests section
  • I added steps for the expected offline behavior in the Offline steps section
  • I added steps for Staging and/or Production testing in the QA steps section
  • I added steps to cover failure scenarios (i.e. verify an input displays the correct error message if the entered data is not correct)
  • I turned off my network connection and tested it while offline to ensure it matches the expected behavior (i.e. verify the default avatar icon is displayed if app is offline)
  • I tested this PR with a High Traffic account against the staging or production API to ensure there are no regressions (e.g. long loading states that impact usability).
• I included screenshots or videos for tests on all platforms
• I ran the tests on all platforms & verified they passed on:
  • Android: Native
  • Android: mWeb Chrome
  • iOS: Native
  • iOS: mWeb Safari
  • MacOS: Chrome / Safari
• I verified there are no console errors (if there's a console error not related to the PR, report it or open an issue for it to be fixed)
• I followed proper code patterns (see Reviewing the code)
  • I verified that comments were added to code that is not self explanatory
  • I verified that any new or modified comments were clear, correct English, and explained "why" the code was doing something instead of only explaining "what" the code was doing.
  • I verified any copy / text that was added to the app is grammatically correct in English. It adheres to proper capitalization guidelines (note: only the first word of header/labels should be capitalized), and is either coming verbatim from figma or has been approved by marketing (in order to get marketing approval, ask the Bug Zero team member to add the Waiting for copy label to the issue)
• If a new code pattern is added I verified it was agreed to be used by multiple Expensify engineers
• I followed the guidelines as stated in the Review Guidelines
• I tested other components that can be impacted by my changes (i.e. if the PR modifies a shared library or component like Avatar, I verified the components using Avatar are working as expected)
• If a new CSS style is added I verified that:
  • A similar style doesn't already exist
  • The style can't be created with an existing StyleUtils function (i.e. StyleUtils.getBackgroundAndBorderStyle(theme.componentBG))
• If new assets were added or existing ones were modified, I verified that:
  • The assets are optimized and compressed (for SVG files, run npm run compress-svg)
  • The assets load correctly across all supported platforms.
• If the PR modifies code that runs when editing or sending messages, I tested and verified there is no unexpected behavior for all supported markdown - URLs, single line code, code blocks, quotes, headings, bold, strikethrough, and italic.
• If the PR modifies a generic component, I tested and verified that those changes do not break usages of that component in the rest of the App (i.e. if a shared library or component like Avatar is modified, I verified that Avatar is working as expected in all cases)
• If the PR modifies a component related to any of the existing Storybook stories, I tested and verified all stories for that component are still working as expected.
• If the PR modifies a component or page that can be accessed by a direct deeplink, I verified that the code functions as expected when the deeplink is used - from a logged in and logged out account.
• If the PR modifies the UI (e.g. new buttons, new UI components, changing the padding/spacing/sizing, moving components, etc) or modifies the form input styles:
  • I verified that all the inputs inside a form are aligned with each other.
  • I added Design label and/or tagged @Expensify/design so the design team can review the changes.
• I added unit tests for any new feature or bug fix in this PR to help automatically prevent regressions in this user flow.
• If the main branch was merged into this PR after a review, I tested again and verified the outcome was still expected according to the Test steps.

Screenshots/Videos

Android: Native

Not recorded yet; this PR is a draft.

Android: mWeb Chrome

Not recorded yet; this PR is a draft.

iOS: Native

Not recorded yet; this PR is a draft.

iOS: mWeb Safari

Not recorded yet; this PR is a draft.

MacOS: Chrome / Safari

Not recorded yet; this PR is a draft.

[github-actions]

⚠️ This PR is possibly changing native code and/or updating libraries, it may cause problems with HybridApp. Please check if any patch updates are required in the HybridApp repo and run an AdHoc build to verify that HybridApp will not break. Ask Contributor Plus for help if you are not sure how to handle this. ⚠️