feat (side panel): add side panel UI skeleton with toggle command

[Harsh16gupta]

This pr adds a Ui panel to the plugin
- A polling-based communication layer between plugin host and webview
- Live progress tracking during the embedding/clustering pipeline
- Results dashboard with strategy comparison and cluster drill-down
- Tab navigation (Dashboard, Change Log, Settings)
- Dark/light theme support via Joplin CSS variables

Screen Recordings:
Dark Mode
https://github.com/user-attachments/assets/871c08ea-f319-45ea-98da-eae7e25d3495
Light Mode
https://github.com/user-attachments/assets/3c50acb6-349f-4b38-b32a-afc2131b6427

[Harsh16gupta]

Hey Bill, this PR is part of a bigger change, I've split it into two PRs to keep things reviewable. This one adds the panel skeleton and UI, and the next one will wire the clustering pipeline to it.

[HahaBill]

Hey Bill, this PR is part of a bigger change, I've split it into two PRs to keep things reviewable. This one adds the panel skeleton and UI, and the next one will wire the clustering pipeline to it.

Hi! From the first look, I recommend using React instead of Vanilla JS because then you can add more frameworks/libraries on top of it. It'll make development better.

[HahaBill]

Hey Bill, this PR is part of a bigger change, I've split it into two PRs to keep things reviewable. This one adds the panel skeleton and UI, and the next one will wire the clustering pipeline to it.

Hi! From the first look, I recommend using React instead of Vanilla JS because then you can add more frameworks/libraries on top of it. It'll make development better.

Especially with functional components

[Harsh16gupta]

Hey Bill, the PR is ready now.
Initially, I planned to split this into two PRs, but since the changes were closely related, I ended up combining them into a single PR.

I have also updated the PR description to reflect the latest changes.

[HahaBill]

Looks good for now :)