Forms: Refactor dashboard layout and add Page components

[lezama]

Proposed changes:

• Refactors the Forms dashboard layout structure by introducing a new Page component system with reusable primitives (Header, Stack, NavigableRegion,
  SidebarToggleSlot)
• Simplifies DataViews footer positioning by removing custom positioning logic and using default sticky behavior
• Wraps the dashboard in SlotFillProvider to enable slot/fill pattern for future extensibility
• Adjusts header offset calculation for mobile viewports to prevent layout issues
• Fixes viewport check logic to properly detect mobile/desktop breakpoints
• Improves overall code maintainability by componentizing layout concerns

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

No

Testing instructions:

Desktop Testing

• Verify the dashboard layout renders correctly
• Scroll down to verify the DataViews footer sticks to the bottom of the viewport
• Click on a response to open the sidebar inspector
• Verify the layout remains stable when opening/closing the inspector

Mobile Testing

• Resize your browser to mobile viewport (< 782px width) or test on actual mobile device
• Verify the header displays correctly without overlapping content
• Verify the DataViews footer positioning is correct
• Verify scrolling behavior works smoothly
• Test opening/closing the inspector sidebar

Multi-select Testing

• Select multiple responses using checkboxes
• Verify the bulk actions UI displays correctly

Expected Results

• No visual regressions compared to trunk
• Layout should be responsive and stable across viewports
• Footer should properly stick to viewport bottom
• No console errors
• Header spacing should be consistent on mobile

[github-actions]

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[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack), and enable the refactor/forms-dashboard-layout-page-components branch.
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bin/jetpack-downloader test jetpack refactor/forms-dashboard-layout-page-components

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[jp-launch-control]

Code Coverage Summary

Coverage changed in 4 files.

File	Coverage	Δ%	Δ Uncovered
projects/packages/forms/src/dashboard/components/response-actions/index.tsx	0/51 (0.00%)	0.00%	2 ❤️‍🩹
projects/packages/forms/src/dashboard/components/export-responses/button.tsx	0/8 (0.00%)	0.00%	1 ❤️‍🩹
projects/packages/forms/src/dashboard/components/response-navigation/index.tsx	0/4 (0.00%)	0.00%	1 ❤️‍🩹
projects/packages/forms/src/dashboard/components/layout/index.tsx	0/9 (0.00%)	0.00%	-5 💚

13 files are newly checked for coverage. Only the first 5 are listed here.

File	Coverage
projects/packages/forms/src/dashboard/components/inspector/body.tsx	0/94 (0.00%) 💔
projects/packages/forms/src/dashboard/components/inspector/mobile.tsx	0/11 (0.00%) 💔
projects/packages/forms/src/dashboard/components/inspector/single.tsx	0/44 (0.00%) 💔
projects/packages/forms/src/dashboard/components/page/header.tsx	0/1 (0.00%) 💔
projects/packages/forms/src/dashboard/components/page/index.tsx	0/3 (0.00%) 💔

Full summary · PHP report · JS report

_{If appropriate, add one of these labels to override the failing coverage check: Covered by non-unit tests Use to ignore the Code coverage requirement check when E2Es or other non-unit tests cover the code Coverage tests to be added later Use to ignore the Code coverage requirement check when tests will be added in a follow-up PR I don't care about code coverage for this PR Use this label to ignore the check for insufficient code coveage.}