Allow image rotation

[andresjmorales]

Allow an image rotation with a button click, similar to the monitor rotation.

This may not be high priority, because like #14 these kinds of things are probably better handled by an image editor prior to uploading. It also seems to make the cropping in Preview view much more difficult.

[andresjmorales]

Issues faced when trying to implement: preview stays black or preview view for some monitors is distorted/stretched, resizing/repositioning rotated image results in weird logic because of adjusted X/Y values, image jumps around... Ultimately maybe out of scope for this app considering how easy image rotation is through OS / other apps.

[andresjmorales]

Try this prompt with a thinking/planning model like Opus 4.6 or Codex 5.3:

# Image Rotation (Exploratory)

## Context
Spanright lets users upload a source image and position/scale it behind monitor rectangles on a Konva canvas (EditorCanvas.tsx). Monitors can already be rotated 90° via a button. Image rotation has been attempted before and ran into issues: preview staying black, distorted/stretched preview for some monitors, and broken drag/resize logic due to rotated coordinate transforms.

## Task
Add 90° increment rotation for the uploaded source image.

## ⚠️ Known difficulties from previous attempts
- Rotated images have swapped effective width/height, which breaks the bounding box calculations in generateOutput.ts
- Drag and resize handlers assume axis-aligned images — rotation transforms the local coordinate space, causing the image to jump or drift when dragged
- The preview panel's per-monitor crop logic needs to account for the rotation when mapping physical coordinates to source image pixels
- These issues compounded and were difficult to isolate

### If implementing, consider this approach
- Do NOT use Konva's rotation transform on the image node. Instead, when the user clicks "Rotate", create a new rotated version of the image using an offscreen canvas:
  1. Create an offscreen canvas with swapped dimensions (width ↔ height)
  2. Rotate and draw the source image onto it
  3. Replace the source image data with this new pre-rotated image
  4. Update the image dimensions in state accordingly
- This way the image on the Konva canvas is always axis-aligned — the rotation is "baked in" to the pixel data
- This avoids all the coordinate transform issues since the rest of the app sees a normal, unrotated image that just happens to have different dimensions

### UI
- Add a rotate button (↻) in the toolbar near the image controls
- Each click rotates 90° clockwise
- Show current rotation state somewhere subtle (tooltip or label)
- Rotation resets when a new image is uploaded

### Scope decision
- If the offscreen canvas approach works cleanly, ship it
- If it introduces new complexity or degrades image quality noticeably, consider closing this issue as "won't fix — use an image editor before uploading" and document that recommendation in the app's help text