feat: tutorial dismiss & reset controls in Preferences UI

[shaoster]

Problem / Motivation

Users who accidentally dismiss a tutorial, or who want to re-run the onboarding flow for a new team member, have no way to reset their tutorial state. The Preferences UI from #318 is the natural home for this control.

Proposed Solution

Extend the Preferences UI (#318) with a "Tutorials" section:

1. "Show tutorials" toggle — maps to a top-level tutorials_enabled boolean in user settings (feat: add per-tutorial dismissed/seen flags to user settings #453). When toggled off, all tutorials are suppressed regardless of per-slug state.
2. "Reset all tutorials" button — clears all tutorials_dismissed flags via a PATCH to the user settings API, restoring the first-login tutorial experience.
3. Per-tutorial status display (read-only): list each tutorial slug with a checkmark if dismissed.

All API calls go through web/src/util/api.ts; async state via useAsync.

Acceptance Criteria

• "Tutorials" section appears in the Preferences UI.
• "Show tutorials" toggle reads from and writes to tutorials_enabled in user settings.
• "Reset all tutorials" clears all tutorials_dismissed flags and shows a confirmation snackbar.
• Per-tutorial dismissed status is displayed (read-only) for each known tutorial slug.
• Covered by unit tests (assert toggle call, reset call, status display).

Dependencies

• Blocked by feat: add Preferences UI for user-level configurations #318 (Preferences UI must exist).
• Blocked by feat: add per-tutorial dismissed/seen flags to user settings #453 (tutorials_dismissed API must exist).
• Should be implemented last in Milestone Piece Creation UI #5, after all tutorial issues are complete.