feat: shared gallery preview in AttachmentPreviewList

[MartinCupela]

Closes REACT-910
Closes REACT-911

🎯 Goal

Bug fix: GlobalModal close order (GlobalModal.tsx)

• Swapped dialog.close() before onClose() in maybeClose. This prevents a throwing onClose callback (like the original e.stopPropagation() on a fake event) from blocking the
  dialog from closing.

Feature: shared gallery preview in AttachmentPreviewList

• AttachmentPreviewList.tsx — now owns a single shared Modal + Gallery for all image/video attachments. Builds galleryItems and a previewIndexById lookup in one memo using
  toBaseImageDescriptors. Each image/video preview receives an openPreview callback that sets the initial slide index and opens the modal. Uses local showPreview state with
  open/onClose props on Modal.
• AttachmentPreviewRoot.tsx — simplified to a pure clickable wrapper. Removed Modal, Gallery, useGlobalModal, and useComponentContext imports. Accepts openPreview callback
  prop; delegates gallery opening entirely to parent.
• MediaAttachmentPreview.tsx / ImageAttachmentPreview.tsx — added openPreview?: () => void prop, passed through to AttachmentPreviewRoot.

Gallery improvements (Gallery.tsx)

• Gallery now resolves GalleryUI from the prop, then ComponentContext, then the default — so context-provided GalleryUI overrides work.

Docs: ComponentContext.tsx

• Updated Gallery and GalleryUI docstrings to clarify the distinction: Gallery is the controller (state + context provider), GalleryUI is the visual presentation.

Other

• MessageComposer.scss — removed an overflow: hidden rule.
• Tests — updated Gallery test for the new default rendering behavior; added test for gallery preview opening from attachment previews.

[github-actions]

Size Change: +460 B (+0.1%)

Total Size: 442 kB

Filename	Size	Change
./dist/cjs/index.js	237 kB	+462 B (+0.2%)
./dist/css/index.css	46.9 kB	-2 B (0%)

ℹ️ View Unchanged

Filename	Size
./dist/audioProcessing-BbOs2wMd.js	1.32 kB
./dist/cjs/emojis.js	2.96 kB
./dist/cjs/mp3-encoder.js	1.27 kB
./dist/css/v2/emoji-mart.css	1.84 kB
./dist/css/v2/emoji-replacement.css	300 B
./dist/css/v2/index.css	39.4 kB
./dist/css/v2/index.layout.css	22.8 kB
./dist/WithAudioPlayback-TERIQpZ6.js	88.3 kB

_{compressed-size-action}

[codecov]

Codecov Report

❌ Patch coverage is 86.04651% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.46%. Comparing base (a9feb11) to head (a913ba6).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
...achmentPreviewList/utils/AttachmentPreviewRoot.tsx	66.66%	3 Missing and 1 partial ⚠️
...er/AttachmentPreviewList/AttachmentPreviewList.tsx	92.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3044      +/-   ##
==========================================
+ Coverage   80.31%   80.46%   +0.14%     
==========================================
  Files         561      561              
  Lines       13098    13120      +22     
  Branches     3413     3419       +6     
==========================================
+ Hits        10520    10557      +37     
+ Misses       1912     1903       -9     
+ Partials      666      660       -6     

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