* feat: add image preview support on /files page

When clicking an image file on the /files tab, show the image using a
signed URL instead of "Preview not available for this file type".

Co-Authored-By: Paperclip <noreply@paperclip.ing>

* feat: add sandbox image preview on /files page

Add image preview support for sandbox (GitHub repo) files on the /files
page. The previous commit only handled Supabase-stored files via
FileInfoDialog. This adds support for the SandboxFilePreview path by:

- Adding getImageContent() to fetch base64 image data via API format=base64
- Extending useSandboxFileContent hook to detect images and return data URLs
- Adding imageUrl prop to FilePreview as alternative to storageKey
- Passing imageUrl through SandboxFilePreview and SandboxFileTree

Depends on API PR #364 (base64 format support for sandbox file endpoint).

Co-Authored-By: Paperclip <noreply@paperclip.ing>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* refactor: remove format=base64 query param, API auto-detects binary files

The API now auto-detects binary files by extension and returns base64
encoding automatically — no need to pass format=base64 in the request.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* refactor: consolidate getImageContent into getFileContents, fix loading UX

Address PR review feedback:
- Eliminate getImageContent.ts — the API auto-detects binary files, so
  getFileContents now handles base64 responses and builds data URLs
- Validate data.content before constructing data URLs
- Fix loading state showing false "Preview not available" for images by
  moving loading/error checks before image/isTextFile checks
- Add a spinner to the loading state

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* refactor: remove storageKey from FilePreview (YAGNI)

storageKey/signed-URL image previews for Supabase-stored files are not
part of this PR's scope. Remove the prop to keep the change focused on
sandbox GitHub image previews only.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* revert: drop formatting-only changes to FileInfoDialogContent

Not part of this PR's scope — keeps the diff focused.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* refactor: extract getMimeType into existing getMimeFromPath utility (SRP)

Add image MIME types to the existing getMimeFromPath utility and reuse
it in getFileContents instead of an inline function.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* refactor: use Skeleton for loading state, move MIME util to lib/files

- Replace loading spinner with shadcn Skeleton component in FilePreview
- Create lib/files/getImageMimeType.ts instead of modifying utils/ (SRP)
- Revert utils/getMimeFromPath.ts to base branch state

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* refactor: move getMimeFromPath from utils/ to lib/files/, add image MIME types

Move getMimeFromPath to lib/files/ per project conventions (no utils/ path).
Add image MIME types (png, jpg, jpeg, gif, webp, svg) to the existing map
so it can be reused for both text and image MIME resolution (DRY).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* refactor: extract FilePreviewSkeleton into its own component (SRP)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: CTO Agent <cto@recoupable.com>
Co-authored-by: Paperclip <noreply@paperclip.ing>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Sweets Sweetman <sweetmantech@gmail.com>