[ui] Improve new index ergonomics

[dskvr]

What problem does this solve?

Adding many indexes is not a hair-pulling, rage inducing experience.

Proposed solution

• Add a filter.
  • Auto-focus filter on "directory picker dialog" open
  • Add keyboard navigation of filter list (arrow keys -> enter key -> new index)
  • Add quick indexing for when the filter has one result (autoselect index:0 result -> enter key)
• Add a way to define a starting directory so it doesn't start from $HOME every time.
• Add an "index" button to each cirectory in the in the file-list visible on:hover. Avoids the unintuitive flow of: entering the directory, seeing an empty directory, moving mouse to bottom of dialog, and clicking add to index.

Alternatives considered

cli (not much better)
grafify (much better)

Confirmations

• I searched existing issues and this is not a duplicate.