Hardware details: replace index-based selectedTrees with stable tree identifiers

## Problem

Hardware pages send selectedCommits as { "0": "head"|hash, … } keyed by the tree’s position in an ordered list. The backend resolves that against ordering in `get_hardware_trees_head_commits`, and the UI mirrors this with treeIndexes / treeCommits[].
Any mismatch in sort order across summary, tabs, caches, or refactors silently maps the wrong commit to the wrong tree.

## Goal

Selections should survive reordering (and differing list order implementations) by identifying trees with a unique key derived from checkout identity (most likely a concatenation of important info (tree name/branch/repo_url, or a key derived of this tuple)).

## Acceptance criteria

* [ ] Selecting trees and pinning commits yields correct data after even if we reorder the tree table (same trees, same selection).
* [ ] Old clients using numeric selectedCommits keys still work for one release.