fix(ui): full-bleed image preview with quiet zoom controls

[kitlangton]

Why

Kit, comparing our image preview with Claude's:

• "Their image just goes the full width. We have some thing that puts a border around it."
• "Can we be more tasteful with this design? I like how we can zoom in."
• On our − 74% + × floating over the top-right of the image: "Can't we make this more tasteful? Should this be floating over it, or not show up until … above it, or in another control strip?"

The preview put the image in a 1200 px panel with its own background, padding and shadow, and laid the controls in a header strip inside that panel.

What Changes

• The image is the preview. No panel: the image fits the window with 56 px top and bottom and up to 48 px at the sides (less on narrow windows). Small images stay at their natural size. Clicking the backdrop around the image closes the preview, as clicking outside the panel did; a click that ends a pan doesn't.
• One quiet control strip under the image. − 74% + │ × sit in a small pill docked at the bottom centre, in the margin under the image, where Claude puts its pager. They never cover the image, and they use the menu surface and floating elevation tokens so they read in light and dark. Focus still starts on Close, and Tab, ⌘ =, ⌘ −, ⌘ 0 and Esc work as before.
• The flight lands on the new box. The rest frame's surface is now the image's own visible box and radius, so the fix(ui): grow the image preview's panel with the image and land without a cut #56997 flight, crop, backdrop and hand-back are unchanged, and the controls no longer ride a panel corner. The panel's shadow becomes a lift shadow under the flying image, deepest mid-flight and gone at both ends, so nothing pops at landing.
• No hard edges when zoomed. A zoomed image fades out over 12 px at the stage edges instead of being cut.
• The window-sized preview doesn't catch clicks. Its root now takes no pointer events; only the open content does. The closing-content and body rules from fix(ui): image preview no longer blocks clicks after closing #57324 stay as they are. Everything unmounts after close (Esc, backdrop, Close, mid-flight).

Placements considered (stills: https://problems.kitlangton.com/media/h_fW9tcxKQb7ihmW-p_Pig.png):

• A. A translucent pill over the image's corner, shown on pointer movement and hidden after 1.5 s idle. It's quiet at rest, but it still covers the image, can't be discovered without moving the pointer, and appears and disappears.
• B. A slim strip above the image. It never covers the image, but it fights the macOS traffic lights and the Windows window controls at the top of the window.
• C (picked). Docked under the image. It never covers the image, nothing appears or disappears, and it matches Claude's pager.

Scope

packages/ui image preview only: image-preview.tsx, image-preview.css and the story's docs. No change to image-flight.ts, zoom math, springs or the open/close timing.

Verification

• Frame-stepped Storybook harness (virtual clock, 60 fps), session timeline under the real sticky title bar, fictional images, Smooth spring on both sides (now the shipped default via fix(ui): image preview no longer blocks clicks after closing #57324):
  • Open, move to the controls, zoom in twice, Esc, at 1×, before | after: https://problems.kitlangton.com/media/6aj21RojPLvkodu7a26yqw.mp4
  • Open and close at 0.25× (small upload open, close and mid-flight reverse, small inline image, large image under the title bar), before | after: https://problems.kitlangton.com/media/WEJHRF8JdADqrPIMZnNaWw.mp4
• No regression of fix(ui): grow the image preview's panel with the image and land without a cut #56997's fixes: the page-background luma ramp is identical frame for frame before and after (22 → 9 on open, 9 → 22 on close), and the large close under the title bar still hands back without a cut (peak per-frame change at the hand-off 24, before 50).
• Light and dark, 480 to 1920 px wide, small, huge, portrait and landscape images: https://problems.kitlangton.com/media/kaUvsIi_n7-31WInTSVrNQ.png
• Playwright checks in the harness: after Esc, a backdrop click, Close and a close mid-flight, no preview or dialog layer stays mounted and the page takes clicks. A click on the image, a click on a zoomed image and a pan don't close it; a backdrop click while zoomed does. Reduced motion only fades.
• fix(ui): image preview no longer blocks clicks after closing #57324's component test passes on this layout: bun run test:components component-tests/image-preview.spec.ts (4 passed).
• packages/ui: bun test src/components/image-flight.test.ts src/components/image-zoom.test.ts src/motion-tuning.test.ts
• bun typecheck in ui, app and session-ui; bun run lint:changed origin/beta is clean.
• CI: build, unit and typecheck pass, and fix(ui): image preview no longer blocks clicks after closing #57324's image-preview component tests pass on Linux and Windows. The e2e reds are the known beta failures (ssh-reconnect, session-question-dock, mobile-timeline-scroll touch) that also fail on fix(desktop): ⌘1–9 switch side-panel tabs inside the side panel #57301.