Improve navigation of large experiment trees (fit view, keyboard pan, search/filter)

[Rexarrior]

Motivation

One of my projects already has about 170 experiment nodes, and the tree is becoming difficult to work with. This is most noticeable in the expanded tree view: moving around requires click-and-drag, and zooming out still does not let me see the entire graph at once. The compact view has the same fit/zoom limitation.

Requested improvements

1. Fit the whole graph. Add a "Fit to view" / "Show all" action and allow zooming out far enough to frame the complete graph. This should work in both expanded and compact views, with the expanded view as the primary use case. An overview need not keep every node label legible; I should be able to see the overall structure, then zoom back in. Please preserve the existing saved-viewport behavior unless I explicitly ask to fit the graph.
2. Keyboard panning. Let the arrow keys move the viewport when the graph is focused, without intercepting keys in search fields or other controls.
3. Search and filter. Let me find an experiment by slug/title, highlight and center a selected result, and narrow the graph by useful criteria (for example, status or subtree) while keeping enough context to understand the relationships between nodes.

For a large research project, finding a node and moving between distant branches should not require repeated drag gestures and guesswork.

Related but distinct: #144 addressed viewport persistence, while #373 discusses moving or archiving experiments. This request is about navigating and reading the existing graph.