Store expanded directory paths in localStorage keyed by workspace path
(key: 'hermes-webui-expanded:{workspacePath}'). On root load (loadDir('.')),
restore the saved set for the current workspace and pre-fetch dir contents
for any restored expanded directories so the tree renders fully on first
paint without requiring a second click to expand.
Saves on every expand/collapse toggle. Switching workspaces automatically
picks up that workspace's own saved state. Per-workspace (not per-session)
so the same tree state is shared across sessions using the same workspace,
which is the natural expectation.
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