Support coloring nodes / clades directly

Empress is great at coloring the tree when there are comparatively few metadata categories, but if there are lots (e.g. coloring a tree by genus-level taxonomy) then it becomes really hard to glean anything from the tree.

It would be nice to be able to select individual tips or internal nodes and assign a specific color (and even text label?) to this node (and optionally the children of the node, if available). This would make it easy to do things like highlight individual genera or species on the tree.

I think this could be achieved by adding a new side panel tab called "Manual Coloring" or something like that, where all of these color assignments are stored in a list (similar to how barplot layers are managed). It would probably be easiest to have this be mutually exclusive with the sample / feature metadata coloring stuff, I guess (so there's a checkbox in this tab for "Color nodes?" or whatever that, upon checking it, undoes the sample / feature metadata coloring).

Review the existing side-panel tabs and barplot-layer management, then trace how sample and feature metadata coloring is enabled. Define the manual-coloring flow for selecting tips or internal nodes, assigning colors and optional labels, and optionally including descendants. Done should include a clear way to manage assignments and switch away from metadata coloring.

Written by the indexing model from the issue text.