Add standalone preview for file suggestion control

[Copilot]

This adds a dedicated frontend/preview entry for the file typeahead used in suggestion.tsx, and removes the standalone-preview dependency on PreviewModel for file suggestion fetching. The preview now uses the existing mock WaveEnv/mock filesystem, starts from ~ by default, and still supports absolute-path queries.

• Extract preview-safe file suggestion fetching

  • Moved the preview file-suggestion request construction into frontend/app/view/preview/previewsuggestions.ts.
  • The helper accepts an explicit cwd/connection instead of reading PreviewModel state directly.
  • frontend/app/view/preview/preview.tsx now uses the extracted helper, preserving existing behavior while making the fetcher reusable from standalone preview code.

• Back preview suggestions with the mock WaveEnv/filesystem

  • Added a mock fetchsuggestions implementation in preview-only code.
  • The mock suggestion path resolution mirrors the expected file-query behavior:
    • default base at ~
    • relative queries like Documents/foo
    • absolute queries starting with /
    • directory navigation/completion via trailing /
  • This plugs into the existing preview WaveEnv narrowing/mocking model instead of introducing a separate ad hoc path source.

• Add a standalone preview

  • Added frontend/preview/previews/suggestion.preview.tsx.
  • The preview renders BlockHeaderSuggestionControl in isolation, anchored to a mock block header, and shows the selected path so the control can be exercised without the full preview view model.

• Tighten the suggestion control plumbing

  • Passed placeholderText through SuggestionControl into SuggestionControlInner so standalone previews display the intended input placeholder.

• Add focused coverage

  • Added preview-level tests for:
    • default ~ anchoring without PreviewModel
    • relative and absolute path query handling
    • disposal flow through the extracted helper

Example of the extracted helper usage:

const fetchSuggestions = (query: string, reqContext: SuggestionRequestContext) => {
    return fetchPreviewFileSuggestions(env, query, reqContext, { cwd: "~" });
};

• Screenshot
  • https://github.com/user-attachments/assets/99f9a2ae-ed26-48be-94c9-26a5b5a029b3

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