feat: tutorial — retroactive entry for previously-made pieces

[shaoster]

Problem / Motivation

Many potters begin using PotterDoc with a backlog of pieces already made. Entering these retroactively requires rapidly advancing through intermediate states and backfilling data — a workflow that is not obvious to new users and easy to do incorrectly (e.g. accidentally leaving required fields blank on intermediate states).

Proposed Solution

Implement the "Retroactive entry" tutorial using TutorialOverlay. Tutorial slug: retroactive_entry.

Steps:

1. Copy: "Have pieces you've already made? You can add them retroactively."
2. Spotlight the New Piece dialog. Copy: "Create the piece as usual."
3. Spotlight rapid state-advance controls. Copy: "Advance through all the states your piece has already passed through — you can fill in field details as you go or come back later."
4. Spotlight the piece history view. Copy: "Use piece history to review past states and correct any field values you entered incorrectly."
5. Copy: "Each state's fields are always editable from the history panel — nothing is locked in."
6. Completion card with Dismiss.

Acceptance Criteria

• Tutorial fires when retroactive_entry is not dismissed (ordering relative to other tutorials is configurable, defaulting to after glaze_fire).
• Each step spotlights the correct element.
• Dismiss marks retroactive_entry via the user settings API.
• Uses TutorialOverlay.
• Covered by unit tests.

Dependencies

• Blocked by overlay shell issue (TutorialOverlay component).
• Blocked by feat: add per-tutorial dismissed/seen flags to user settings #453 (dismissed/seen flags API).