feat(side-sheet): support default width

[Noggling]

Component Development

Type

• Feature
• Fix
• Hotfix (should release ASAP)
• Maintenance (deps only)

Reference to assignment

Closes equinor/fusion-core-tasks#1871

Parent task: equinor/fusion-core-tasks#1847

Description of assignment

Allow consumers to choose the width at which a side sheet initially opens without changing the existing minimum-width behavior or requiring a controlled width API.

Description of Proposed Changes

• Add an optional defaultWidth prop accepting pixel numbers or viewport percentages such as "50%".
• Preserve existing behavior by falling back to minWidth, then 480px.
• Keep resizing functional by storing the measured pixel width after a resize.
• Add a Storybook control and example for testing the opening width.
• Add a minor changeset for @equinor/fusion-react-side-sheet.

Checklist

Create User Story(DevOps) or Issue(Github)

• Describe functional requirements
• Design plan/suggestion (e.g. images, link to eds design or relevant figma links, mention of eds tokens to use)
• Communication with UX (evaluate if needed)

Development

• Describe the component
• Code funtional requirements
• Create stories
• Create snapshots (evaluate if needed)
• Remove unused imports and commented code
• Lint code with Biome (blocked by the existing local Biome CLI/config schema mismatch)

Create Pull Request

• Assign relevant reviewers in Github and Cromatic
• Inform colleagues of pending reviews

Review of Pull Request

• Follow up on comments from code review and implement necessary changes (evaluate if needed)
• Review approved in github (and chromatic)

Publish Code

• Add changeset
• Merge PR
• Consider letting fusion developers know this feature exists

Verification

• npm run build --workspace=@equinor/fusion-react-side-sheet
• bun run storybook:build
• Browser-verified defaultWidth="50%" renders at 785px in a 1570px viewport.

[changeset-bot]

🦋 Changeset detected

Latest commit: 4304193

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@equinor/fusion-react-side-sheet	Minor

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