Outsource custom GUI class components to dashgui library

Move reusable GUI primitives (PanelTile, EditablePanelTile, base Tile,
ElevatedPill, SingleLineTextEdit) into the shared dashgui package and
consume them via a file: dependency. EditablePanelTile now accepts an
onError prop so error notification stays app-side.

README.md

@@ -27,6 +27,19 @@ To test the full PWA precache (production build + preview, no HMR), use
 Dashback must be running and your Google account must be authorized on the
 backend.
 
+## Shared GUI library (`dashgui`)
+
+Reusable custom GUI class components live in the **`dashgui`** package
+(`../dashgui`), installed here as a `file:` dependency (npm workspace symlink)
+so source imports are reflected instantly.
+
+- `dashgui/panels` — `PanelTile`, `PanelTileGrid`, `EditablePanelTile`, layout helpers
+- `dashgui/tiles` — base `Tile` (extended by `src/components/tiles/*`)
+- `dashgui/SingleLineTextEdit`, `dashgui/ElevatedPill`
+
+See `../dashgui/README.md` for details and the dev/mock-page harness used for
+future test cases.
+
 ## Auth flow
 
 1. On boot the app calls `GET /api/auth/me` with `credentials: 'include'`.