[Webtoon study] Add transition and interstitial scene editor

[realproject7]

Context

The samples show that transitions are often not normal illustrated cuts. Webtoons use white space, black fades, gradients, object inserts, door/window/phone shots, text-only panels, SFX-only panels, and time/location cards to control scroll rhythm and imply time passing.

Required Changes

Add a first-class transition/interstitial editor for cartoon episodes. It should allow users and agents to create scene connectors between image cuts:

• White gutter with adjustable height.
• Black gutter or dark gradient for dread/scene break.
• Time skip card.
• Location card.
• Text-only caption card.
• SFX-only card.
• Object insert card: phone, document, button, clock, message, door, sign, etc.
• UI insert card: chat/message/call/status/system screen.
• Reaction pause card: small face/detail image plus large surrounding whitespace.

Data Model

Represent these as episode elements, not fake image cuts:

• type: imageCut | transition | textPanel | uiInsert | objectInsert | sfxPanel
• heightPolicy
• backgroundStyle
• textBlocks[]
• sfx[]
• linkedBeatId
• beforeCutId / afterCutId

UX Requirements

• In cut review, between-scene slots should be visible and editable.
• In focused editor, a transition element should open a full-height editor with background/spacing/text controls.
• Transition cards should be previewed in sequence exactly as exported.

Acceptance Criteria

• Users can add/edit/delete/reorder transition elements between cuts.
• Export includes transition/interstitial elements in the final image sequence.
• Publish readiness accounts for transitions as intentional content, not missing art.
• Agents can request a transition element instead of forcing every story beat into an illustration.

Verification

• Tests for transition persistence and export ordering.
• Browser check that preview shows transitions between image cuts.
• Original sample episode generated in OWS uses at least three transition types.