feat: make object preview dialogs resizable

[cxymds]

Pull Request

Description

File previews previously used a fixed dialog size. This change lets users drag the preview corner on desktop or use Expand/Collapse to choose a comfortable reading area.

• Keep the dialog anchored while dragging, constrain it to the viewport, and restore its previous manual size after collapsing an expanded view.
• Make text, image, PDF, Parquet, TIFF, and sandbox previews follow the available space; refit images when the preview container changes size.
• Preserve compact defaults for audio and unsupported files, with keyboard-accessible scrolling, localized controls, and touch-friendly buttons.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• Performance improvement
• Test improvements
• Security fix

Testing

• Unit tests added/updated
• Manual testing completed

All required checks passed in an isolated worktree based on main:

nvm use v22
pnpm install --frozen-lockfile
pnpm type-check
pnpm lint
pnpm format:check
pnpm test:run
git diff --check

• Full suite: 489 passed, 0 failed, including 13 preview dispatch/source checks.
• Chromium interaction testing with synthetic fixtures covers pointer resizing, viewport bounds, keyboard Expand/Collapse with manual-size restoration, PageDown scrolling, image refitting, and mobile layout.
• At a 1280×720 viewport, the dialog resized from 896×612 to 1036×640, expanded to 1248×688, and collapsed back to 1036×640. At 390×844, it remained within the viewport with 44×44 touch controls.
• Independent correctness, testing, UX/accessibility, and simplification reviews completed.

Checklist

• Code follows the project's style guidelines
• Self-review completed
• TypeScript types are properly defined
• All commit messages are in English (Conventional Commits)
• All existing tests pass
• No new dependencies added, or they are justified

Related Issues

Implements the requested resizable file-preview window. No linked GitHub issue.

Screenshots (if applicable)

All captures use the English UI and synthetic English fixture content rendered by the actual preview component. The temporary fixture routes are not included in this PR.

Desktop: before and after (1280×720)

Before: fixed dialog	After: resizable dialog

Desktop: resizing controls

After dragging the corner	After keyboard Expand

Mobile: before and after (390×844)

Before	After	After Expand

Desktop dark mode

Image refitting after resizing

Default dialog size	Smaller dialog after dragging

The screenshots are hosted at an immutable commit on the separate evidence branch. They are not included in this PR's changed files.

Additional Notes

• Native corner resizing is enabled for desktop fine-pointer devices; Expand/Collapse is also available on touch and narrow screens.
• No new dependencies, backend changes, or edits to the shared UI primitives.
• Automated preview checks cover dispatch and source structure; Safari/Firefox and end-to-end loading of real PDF, Parquet, and TIFF files were not tested.
• The image fixture displayed correctly and refitted after resizing; the development console reported Next Image initial-height and LCP warnings during loading. Live S3 integration was not tested.