Moves Explorer’s back navigation and create actions into a reusable
sidebar breadcrumb header. Reduces product-menu headings globally to
`text-sm` and keeps breadcrumb links free of padding, borders, and
backgrounds.
### How to test
1. Open `/project/<ref>/explorer` and confirm the header shows Explorer
and the SQL Editor switch action.
2. Open Notebooks and Chats. Confirm the header shows `Explorer >
Notebooks/Chats` and the corresponding create action works.
3. Return using the Explorer breadcrumb with a click or Tab + Enter.
Check that the label stays aligned and has no hover background.
4. Open another product, such as Database, and confirm its sidebar
heading uses the smaller font size.
5. At a mobile viewport, open the menu and repeat the notebook/chat
actions and back navigation without closing the sheet. Confirm the
header stays current and disappears when returning to the main menu or
opening another product.
Validation: 18 focused tests, typecheck, formatting, and lint ratchet
passed.
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
- **New Features**
- Added a shared Explorer sidebar header with breadcrumbs and contextual
actions for creating notebooks and chats.
- Added keyboard-accessible navigation between the Explorer overview and
notebook or chat sections.
- Added support for customized product menu headers across project
layouts.
- **Improvements**
- Centralized Explorer navigation and actions in the shared sidebar
layout.
- Improved mobile menu updates when navigating between Explorer
resources.
- Refined Explorer home layout and drag-handle behavior across screen
sizes.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
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Co-authored-by: Joshen Lim <joshenlimek@gmail.com>
