ENH: Post to Elog from snapshot details

[YektaY]

Description

• Adds a "Post to Elog" button on the snapshot details page, gated on the
  backend's /v1/elog/config feature flag (hidden when no adapter is
  configured).
• Opens a dialog with logbook and tag pickers (fetched from the backend),
  an editable title, and a markdown body prefilled with the snapshot
  metadata plus a table of selected PVs. Edit/Preview tabs via
  react-markdown.
• Backend-stamped author — the UI just posts the markdown; attribution
  comes from the API key on the backend.

Depends on [backend PR]

Motivation

Had a request from NLCTA to be able to make posts to the e-log containing snapshot data.

Note: this current pass only partially addresses NLCTA's request, they want to be able to follow up an entry as well as create a new entry from Squirrel. Will need to look at the E-log API and change how the dialog form works to accommodate that functionality

Closes #89.

Where Has This Been Documented?

Note: documentation still needed

Screenshots

Note: need to add screenshots

Pre-merge checklist

• Code works interactively
• Code follows the style guide
• Code contains descriptive docstrings
• New/changed functions and methods are covered in the test suite where possible
• Test suite passes locally
• Test suite passes on GitHub Actions