F13 — CRT display effect (non-destructive view effects)

[Flow-Fly]

Lifted from #72 (F13). Pure delight / "fun to use" pillar of the product.

Goal

A display-only CRT effect (scanlines, phosphor mask, barrel curvature, bloom, vignette) over the composited output — never touching pixel data, cels, or the project file. Same principle as the preview background modes from #52.

Architecture decision (from the PRD)

Build a generic post-process view-effect slot, not a hardcoded CRT toggle — future effects (LCD grid, Game Boy palette preview) reuse the slot. Effect settings live in the settings store (src/stores/settings.ts), never in ProjectFile.

Surfaces, in order

1. Live preview overlay (src/components/preview/pf-preview-overlay.ts) — cheapest, ship first.
2. Main viewport "view mode: CRT" — optional second step.
3. Export "with effect" — baked into GIF/WebP/PNG copies, clearly labeled, at export time only.

Sub-issues / order

1. View-effect slot infrastructure (WebGL post-process pass over a composited canvas)
2. CRT shader + presets + settings UI
3. Export-with-effect + main viewport view mode

Acceptance (feature-level)

• Toggle CRT on the preview, tune intensity; save/load hash of project data is unchanged.
• Export "with effect" produces a separate styled copy; normal export untouched.

Dependencies

None hard. Synergy: F5 share pages may offer it as a viewer toggle; F14 reveal moment uses it.

[Flow-Fly]

F13 is shipped on main through PRs #296 and #299, and the owner has accepted the current implementation. Closing the feature and its completed children. Future visual refinement is tracked separately in deferred issue #301.