refactor(web): use css icons for PDF preview

[lyzno1]

Summary

• replace the PDF preview zoom-out, zoom-in, and close React SVGs with CSS icons
• keep all three glyphs decorative with aria-hidden
• remove the three exact icon lint warnings without changing control semantics

Boundary

• one production component only
• no dialog containment, zoom math, hotkeys, callbacks, positions, button labels, or tests changed

Validation

• pnpm --dir web exec vp test run app/components/base/file-uploader/__tests__/pdf-preview.spec.tsx — 10/10 passed
• ./node_modules/.bin/vp lint web/app/components/base/file-uploader/pdf-preview.tsx web/app/components/base/file-uploader/__tests__/pdf-preview.spec.tsx
• pnpm --dir web lint:a11y app/components/base/file-uploader/pdf-preview.tsx
• pnpm check — 0 errors, 2056 warnings, exactly 3 fewer than the parent
• git diff --check

Visual regression review

• compare the zoom-out, zoom-in, and close glyph shapes
• verify every icon remains 16x16 and keeps the same gray color
• verify the controls stay at right-24, right-16, and right-6 with the same 32px hit areas
• verify tooltip labels and keyboard activation are unchanged

Stack

• depends on refactor(web): rely on preview dialog containment #40363 because both PRs touch the PDF preview owner
• this visual-only leaf can be reviewed or reverted independently

Rollback

Revert commit 00aba9a51ff7e123b9adc4802b4056f2dd646b7d; the parent containment cleanup remains intact.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.43%. Comparing base (d93c661) to head (00aba9a).

Additional details and impacted files

@@                            Coverage Diff                             @@
##           codex/refactor-preview-dialog-containment   #40364   +/-   ##
==========================================================================
  Coverage                                      86.43%   86.43%           
==========================================================================
  Files                                           5107     5107           
  Lines                                         290736   290736           
  Branches                                       57924    57924           
==========================================================================
  Hits                                          251289   251289           
  Misses                                         34506    34506           
  Partials                                        4941     4941           

Flag	Coverage Δ
dify-ui	93.49% <ø> (ø)
web	86.11% <ø> (ø)

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[lyzno1]

Closing during the accessibility stack audit. This layer only changes component typing or icon implementation, adds no user-visible accessibility contract, and carries independent visual/review risk. It can be reconsidered later as a separate cleanup, not as an a11y dependency.