## Context
More groundwork for the Explorer - initializing the home page
Note that nothing here is functional, all just visual still
<img width="1387" height="960" alt="image"
src="https://github.com/user-attachments/assets/d4967578-edbd-476f-8150-d9d5e9d66666"
/>
<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit
* **New Features**
* Added a new Explorer landing page with an assistant chat form.
* Added quick actions for creating notebooks and SQL work.
* Added notebook and chat template cards for faster project exploration.
* **Improvements**
* Explorer content now fills the available page height.
* Assistant send button styling now reflects whether submission is
available.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
## Context
Resolves FE-4074
Just adds scaffolding for the explorer UI - no data fetching yet.
Initializes the page + side nav, based off Saxon's POC in
`poc/explorer-prototype`
<img width="1389" height="500" alt="image"
src="https://github.com/user-attachments/assets/8f293992-97d9-403e-91d6-2e104cd20eb5"
/>
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
## New Features
- Added a project Explorer page accessible from
`/project/:ref/explorer`.
- Added navigation for browsing notebooks and chats.
- Added search fields, back navigation, animated transitions, and empty
states for Explorer sections.
- Added a conditional Explorer link to the SQL Editor menu when enabled.
## Documentation
- Marked the Explorer route migration as complete in the migration
checklist.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
Joshen Lim