feat(viewer): edit and save Views in the embedded Neuroglancer

[allison-truhlar]

Editable-Views branch 13: a View's owner edits it in the embedded Neuroglancer viewer (/view/<read_key>) and keeps the changes with an explicit Save or Discard. This builds on #458 (relink UI) and the existing PUT /api/neuroglancer/views/{short_key}.

Editing reads the iframe's window.viewer, so it works only when Fileglancer and Neuroglancer share an origin. With a cross-origin Neuroglancer, such as the CLI default neuroglancer-demo.appspot.com, the owner sees "Editing isn't available with this Neuroglancer deployment" and the viewer stays read-only.

What changed

• Viewer bridge: the new useNeuroglancerViewer hook polls the iframe for window.viewer. It reports ready for a same-origin viewer and unavailable when access throws, and it exposes getState, setState, and change subscriptions.
• Dirty tracking: useViewEditState compares the viewer's toJSON() with a saved baseline. Until the user first interacts, each change resets the baseline, so Neuroglancer's own normalization and position updates during load don't count as edits. After a Save, the baseline is the state the request sent, so a camera move during an in-flight Save stays unsaved.
• Save bar: when the owner has unsaved changes, a bar reads "Unsaved changes — saving updates this View for everyone with its link." and offers Discard and Save. A failed Save keeps the changes and shows an error toast. The viewer sets the iframe src once, at first load, so the refetch after a Save doesn't reload Neuroglancer.
• Broken-sources banner: the owner sees each broken, relinkable source once per dataset, with a Relink button that opens RelinkDialog from feat(views): relink broken View sources from the Views table and on Data Link create #458. After a relink, the bridge pushes the new state into the viewer; when the bridge is unavailable, the iframe reloads with the relinked state. The Relink button stays disabled while the View has unsaved changes, with the tooltip "Save or discard changes first".
• Unsaved-changes guard: with unsaved changes, in-app links (logo, Views breadcrumb, profile menu) and the Copy link, Download JSON, and Open in Neuroglancer actions open a dialog with Save and continue, Continue without saving, and Cancel. Exports use the state that Save sent, or the last saved state if the owner continues without saving. Closing or reloading the tab triggers the browser's beforeunload prompt. BrowserRouter has no useBlocker, so a capture-phase click listener catches same-origin links before React Router handles them. Browser back and forward still leave without asking.
• Reload warning: if Neuroglancer reloads while the View has unsaved changes, a toast warns that the changes may be lost, and the Save bar hides until the viewer is ready again. Neuroglancer restores its URL hash, so edits can survive the reload. The viewer then compares with the saved baseline instead of resetting it.
• Owner only: every edit control reads useCanEditView. A non-owner gets no Save bar, note, banner, dialog, or leave prompt.
• Dev-only Neuroglancer passthrough: the new neuroglancer_url setting (FGC_NEUROGLANCER_URL) serves that Neuroglancer at /neuroglancer/, so you can test editing locally. The setting is unset by default, and the routes exist only when you set it. The passthrough is a GET proxy that rejects dot segments, %, ?, and # in the path. Production serves Neuroglancer from its reverse proxy instead. The Vite dev server proxies /neuroglancer to the backend. For setup, see "Testing View editing locally" in docs/Development.md.

Testing

The backend suite (1122) and frontend suite (517) pass, ESLint and Prettier are clean, and node-check stays at the known 5-error baseline.

I tested editing by hand against the real Neuroglancer through the dev passthrough:

• Opening a View and touching nothing doesn't mark it as unsaved
• Opening a View and changing something shows the "unsaved changes" bar with a button to save. Clicking the save button shows a success toast (if successful; an error toast if not). Reloading or reopening the View from the Views page keeps the new saved View.
• Clicking the "Discard" button in the "unsaved changes" bar causes the viewer to jump back to the saved state.
• Attempting to navigate back to the Views page while there are unsaved changes brings up a dialog with options to save and continue, discard and continue, or cancel. Tested all three.
• Opening a View when the same-origin Neuroglancer is not available (e.g., via the dev passthrough) shows a banner at the top that editing isn't available.

@krokicki