Docs updates for E/A#99912 Home Concierge prompt box mention and emoji suggestions

[MelvinBot]

Explanation of Change

Help site follow-up for #99912, which added @mention and :emoji suggestions to the Concierge prompt box on Home and fixed suggestions rendering behind the right-hand Concierge panel.

The Concierge prompt box on Home was not documented on the help site at all, so there was nowhere for the new suggestion behavior to live. This adds it to docs/articles/new-expensify/getting-started/Expensify-Home-Overview.md:

• Lists the Concierge prompt box in What Home shows.
• Adds How to ask Concierge a question from the prompt box on Home — what the box is, its placeholder copy on wide and narrow screens, how to attach a file, how to send, and that unsent text is kept as a draft.
• Adds How to mention someone in the Concierge prompt box on Home — typing @, selecting a suggestion, and the web keyboard behavior (arrow keys, Enter / Tab to insert, Escape to dismiss, prompt not sent while the list is open).
• Adds How to add an emoji in the Concierge prompt box on Home — typing : and inserting a suggestion.
• Adds FAQ entries for "my prompt didn't send when I pressed Enter", using the same suggestions in the Concierge panel, and unsent drafts. Updates the "Can actions be completed from Home?" answer, which previously said no actions can be taken from Home.
• Updates keywords and internalScope to match.

Every UI string in the article was checked against the running app on web rather than taken from the code: the wide-screen placeholder Ask Concierge to analyze your expenses or get support, the narrow-screen placeholder Ask Concierge anything, the Add attachment menu item, the Send icon, and the top-right Help icon that opens Concierge in the panel on the right.

No other help site article needed changes — the rest of the PR (the useComposerSuggestions extraction, the suggestions portal z-index, and the viewport-bottom helpers) is internal refactoring with no user-facing documentation impact.

One screenshot is suggested as a non-rendered <!-- SCREENSHOT: --> placeholder, per docs/HELP_AUTHORING_GUIDELINES.md section 8.

Fixed Issues

$ #99550
PROPOSAL: N/A

Tests

// TODO: The human co-author must fill out the tests you ran before marking this PR as "ready for review"
// Please describe what tests you performed that validates your changed worked.

Suggested review (docs-only change, no app code):

1. Read the rendered article and confirm each described behavior matches the app on web and mobile.
2. Confirm the quoted UI strings still match the current UI.

• Verify that no errors appear in the JS console

Offline tests

QA Steps

// TODO: The human co-author must fill out the QA tests you ran before marking this PR as "ready for review".
// Please describe what QA needs to do to validate your changes and what areas do they need to test for regressions.

This PR only changes help site markdown under docs/articles. No app behavior changes.

• 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

Android: mWeb Chrome

iOS: Native

iOS: mWeb Safari

MacOS: Chrome / Safari

Reference captures used to verify the documented behavior and UI strings against the running app on web (not part of the docs change itself).

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@GCyganek, please review the files changed and confirm they reflect the current behavior. Then post a new comment saying @MelvinBot mark this PR as ready for review.