docs: organize tool pages by modality under /tools/<modality>/

Move all 157 tool pages into image/video/audio/pdf/data subfolders so URLs read /tools/<modality>/<id> (e.g. /tools/image/crop). Nest the image sub-categories under an Image group in the sidebar so the nav reads by modality. Add public/_redirects (301, clean + .html forms) mapping every old flat /tools/<id> URL to its new path so inbound links keep working. Rewrite all internal /tools links. Verified with a clean docs build (no dead links).

apps/docs/tools/image/compare.md

@@ -0,0 +1,65 @@
+---
+description: Compare two images side by side with pixel-level diff visualization and similarity score.
+---
+
+# Image Compare
+
+Upload two images to compute a pixel-level difference map and a numerical similarity percentage. The output is a diff image highlighting changed regions in red.
+
+## API Endpoint
+
+`POST /api/v1/tools/image/compare`
+
+Accepts multipart form data with **two** image files. No settings field is needed.
+
+## Parameters
+
+This tool has no configurable parameters. Upload exactly two image files.
+
+| Field | Type | Required | Description |
+|-------|------|----------|-------------|
+| file (first) | file | Yes | The first image |
+| file (second) | file | Yes | The second image |
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/image/compare \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@original.jpg" \
+  -F "file=@modified.jpg"
+```
+
+## Example Response
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "similarity": 94.52,
+  "dimensions": { "width": 1920, "height": 1080 },
+  "downloadUrl": "/api/v1/download/a1b2c3d4-e5f6-7890-abcd-ef1234567890/diff.png",
+  "originalSize": 4900000,
+  "processedSize": 280000
+}
+```
+
+## Response Fields
+
+| Field | Type | Description |
+|-------|------|-------------|
+| jobId | string | Job identifier for downloading the diff image |
+| similarity | number | Percentage similarity between the two images (0 to 100) |
+| dimensions | object | Width and height used for comparison |
+| downloadUrl | string | URL to download the generated diff image |
+| originalSize | number | Combined size of both input images in bytes |
+| processedSize | number | Size of the diff output image in bytes |
+
+## Notes
+
+- Both images are resized to the same dimensions (the maximum of each axis) before comparison.
+- The diff image highlights differences in red with opacity proportional to the magnitude of change. Identical or near-identical pixels (difference < 10) are shown as semi-transparent versions of the original.
+- Similarity is calculated as the inverse of the average pixel difference across all pixels, expressed as a percentage.
+- A similarity of 100% means the images are pixel-identical (at the comparison resolution).
+- The diff output is always PNG format regardless of input formats.
+- Both images are validated and decoded (HEIC, RAW, PSD, SVG supported) before comparison.
+- EXIF orientation is auto-applied on both images before processing.