[PDF Ink Signatures] Side panel polish

Update the side panel to match the latest UX spec.
- Updating various spacing
- Changing select dropdown styles to the standard .md-select styling in
  dark mode. Refactoring these .md-select styles into pdf_shared.css, so
  that the bottom toolbar can use them also.
- Removing "justify" alignment option
- Also making small adjustments to the bottom toolbar. There is no mock
  for this yet, so making minimal changes required for it to look
  visually reasonable with the adjustments made for the side panel.

Bug: 402547554
Change-Id: I14b42ec824c60a0d7a1ee4af36b7cf119d59b09b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6543809
Reviewed-by: Andy Phan <andyphan@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1460228}