fix(pdf): Rotated photo frames print at the angle they preview at

[patriksimms]

Problem

A rotated image frame exported axis-aligned. Text already rotated, so a rotated
layout printed with its words at an angle and its pictures square.

Solution

Photos, decorative images, and empty-slot filler art are now drawn inside a
transformation matrix that turns them around the frame's top-left corner — the
corner the preview turns an element around (transform-origin: top left). Page
space counts Y upwards where CSS counts it downwards, so the preview's angle is
negated in the export.

pdf-lib's per-call rotate option was not usable here: it turns each drawing
around its own anchor, which moves an element instead of turning it in place and
would pull a gallery's slots apart. A gallery therefore embeds its photos before
anything is painted, so one transformation can wrap the whole frame.

In scope

• Photos in image frames and gallery slots
• Decorative image elements
• Filler art in photo slots a contributor left empty

Out of scope

• Rotated rectangles, circles, and lines, which pdf-lib still turns around their
  bottom-left corner. Every shipped background preset is deliberately
  axis-aligned, and fix(pdf): Rotated frames export axis-aligned instead of rotated #86 scoped this fix to picture content.

Acceptance criteria

Criterion	Evidence
A rotated image frame exports at the angle and place it previews at	crops a photo to the frame at the angle the preview shows
A rotated gallery frame rotates as a unit, slots in the same relative places	rotates a gallery as one frame, keeping its slots in the same relative places
A rotated decorative image exports at the angle it previews at	turns a decorative image at the angle the preview shows
A rotated empty photo slot's filler art rotates with its frame	turns filler art with the frame, so a slot does not move when a photo arrives
An axis-aligned frame is untouched	leaves an unrotated frame exactly where it was
An exported-PDF proof shows a rotated frame with and without a photo landing in the same place	screenshots below

The new tests replay the exported page's drawing operators and compare
page-space corners, so they assert where a photo actually lands rather than
which operators pdf-lib happens to write. All four rotation tests fail against
main and pass here; the axis-aligned test passes both ways.

Exported-PDF proof

Rasterised from an exported PDF of the same layout: an image frame at -12°, a
two-slot gallery at 8°, a decorative image at 15°, and a caption at -12° for
reference.

Rotated frames holding photos

Before	After

The same frames with no photo uploaded

Before	After

Validation

• bun run format:check, bun run lint, bun run typecheck
• bun run test — 349 passed; the 14 failures need the local Postgres and S3
  services, which are unavailable in this environment and fail identically on
  main
• bun run build with the CI Clerk test keys
• bun run test:e2e not run locally (no services or browsers); left to CI

No configuration outside the repository is required.

Closes #86