Shared color scope: reuse type/track colors across datasets

[mattdawkins]

Summary

Type/track/group color and style overrides were stored only in the metadata of the dataset they were set on (customTypeStyling / customGroupStyling). The same class could therefore render a different color in every sequence, and a color a user picked never followed them to the next dataset.

This adds a Type color scope user setting (User Settings dialog) with two modes:

• Shared across all data (default) — one set of color/style overrides is reused across every dataset.
  • Desktop (no user accounts): stored once per data directory in global_style_settings.json, shared across all sequences.
  • Web: scoped to the current user/browser via localStorage, consistent with how the web client already persists user preferences (clientSettings).
  • On dataset open, the shared styles overlay the dataset's own styling (shared wins on conflicts); any style the user edits is mirrored back to the shared store; and colors that ride in with a dataset seed the shared store (without overwriting existing shared choices) so imported colors propagate to future sequences.
• Per dataset — the original behavior; colors are saved only with the dataset they were set on.

The default is shared, so out of the box a color set once is reused everywhere. Choosing Per dataset reproduces today's exact behavior (the shared paths become no-ops).

Changes

• dive-common/store/settings.ts — new typeSettings.colorScope: 'shared' | 'dataset' (default 'shared'; hydrates onto existing stored settings).
• dive-common/components/UserSettingsDialog.vue — a "Type color scope" select.
• src/StyleManager.ts — optional onStyleEdit hook, fired on updateTypeStyle (genuine edits only, not on dataset load).
• dive-common/components/Viewer.vue — loads/overlays/seeds the shared store on dataset open and persists edits (debounced).
• dive-common/apispec.ts — new GlobalStyleSettings type and optional loadGlobalStyleSettings / saveGlobalStyleSettings Api methods.
• Desktop: backend/native/globalStyles.ts (file read/write under settings.dataPath, degrades to empty on missing/corrupt), IPC handlers, and frontend api wrappers.
• Web: localStorage-backed implementation in web-girder/api/dataset.service.ts, wired into provideApi.

Test plan

• vitest run platform/desktop/backend/native/globalStyles.spec.ts → 6 passed (round-trip, missing dir, corrupt file, normalization).
• vitest run src/StyleManager.spec.ts → passed (optional hook is additive).
• eslint on all changed files → clean.
• electron-vite build (desktop backend + renderer) and vite build (web) → both succeed.

Notes

• Web persistence is per-user-per-browser (localStorage), not cross-device — there is no per-user server settings blob today, and adding one would need a new Girder route. This keeps the change client-only; a server-backed per-user store can be a follow-up.
• Group styling is included alongside type styling for completeness.