Agentic UI: Foundation

[shaunandrews]

This PR aims to improve the new Agentic UI for Studio, including the visual balance, user experience, and usability. Its not meant to be a complete solution; there are still many rough edges and broken flows. But this PR serves as a foundation for a series of other PRs and future work, including: #3717 #3737 #3738

Proposed Changes

• Increased base UI font-size to 14px.
• Drag handles for panels (like the sidebar menu and the browser preview) are now integrated into the border for the panel, no longer affecting the layout.
• Added smoother panel behavior, including animated site preview open/close and improved split-view layout.
• Removes the placeholder "Dashboard" and "Skills" menus and screen.
• A redesigned sidebar menu includes larger site icons, compact timestamps, clearer active states, consistent action placement, running-chat indicators, and an Unassigned group for chats that don't belong to a site.
• Clicking on a site in the sidebar shows a new site detail screen. Click the site avatar to manually toggle all active chats for a site.
• Sidebar chats now filter archived conversations out of the regular list.
• Added site overview and unassigned overview routes with active and archive chat lists, and a way to start a new chat.
• Studio's browser gets buttons for back, forward, and refresh, a page title display, and moves the annotation feature into the toolbar. The browser is also now shown by default. The OS View menu includes a toggle, and users can still use the keyboard shortcut to toggle.
• You can now add images to your chat messages.
• Included follow-up compatibility and hardening fixes for React 19 / @wordpress/ui, Base UI submenu behavior, markup sanitization, and CodeQL feedback.

How to test

Checkout this PR, build the app, and then use the Electron (Studio) -> Beta Features menu to enable the Agentic UI.

Screenshots

[wpmobilebot]

📊 Performance Test Results

Comparing 48ec056 vs trunk

app-size

Metric	trunk	48ec056	Diff	Change
App Size (Mac)	1384.37 MB	1384.46 MB	+0.08 MB	⚪ 0.0%

site-editor

Metric	trunk	48ec056	Diff	Change
load	1704 ms	1643 ms	61 ms	🟢 -3.6%

site-startup

Metric	trunk	48ec056	Diff	Change
siteCreation	9576 ms	9547 ms	29 ms	⚪ 0.0%
siteStartup	4422 ms	4423 ms	+1 ms	⚪ 0.0%

ui-classic-panels

Metric	trunk	48ec056	Diff	Change

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Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)