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/border.md

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+# Border & Frame
+
+Add borders, padding, rounded corners, and drop shadows to images. The tool applies effects in order: padding, border, corner radius, then shadow.
+
+## API Endpoint
+
+`POST /api/v1/tools/image/border`
+
+## Parameters
+
+| Parameter | Type | Required | Default | Description |
+|-----------|------|----------|---------|-------------|
+| borderWidth | number | No | 10 | Border thickness in pixels (0 to 2000) |
+| borderColor | string | No | `"#000000"` | Border color as hex (e.g. `#FF0000`) |
+| padding | number | No | 0 | Inner padding between image and border in pixels (0 to 200) |
+| paddingColor | string | No | `"#FFFFFF"` | Padding fill color as hex |
+| cornerRadius | number | No | 0 | Corner radius in pixels (0 to 2000) |
+| shadow | boolean | No | `false` | Whether to add a drop shadow |
+| shadowBlur | number | No | 15 | Shadow blur radius (1 to 200) |
+| shadowOffsetX | number | No | 0 | Shadow horizontal offset (-50 to 50) |
+| shadowOffsetY | number | No | 5 | Shadow vertical offset (-50 to 50) |
+| shadowColor | string | No | `"#000000"` | Shadow color as hex |
+| shadowOpacity | number | No | 40 | Shadow opacity percentage (0 to 100) |
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:13490/api/v1/tools/image/border \
+  -F "file=@photo.jpg" \
+  -F 'settings={"borderWidth":20,"borderColor":"#333333","cornerRadius":16,"shadow":true,"shadowBlur":25,"shadowOpacity":50}'
+```
+
+## Example Response
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "downloadUrl": "/api/v1/download/a1b2c3d4-e5f6-7890-abcd-ef1234567890/photo.png",
+  "originalSize": 456789,
+  "processedSize": 523456
+}
+```
+
+## Notes
+
+- Uses the standard `createToolRoute` factory. Accepts a single image file via multipart upload.
+- Supports HEIC, RAW, PSD, and SVG input formats (automatically decoded).
+- Processing order: padding is added first, then the border wraps around, then corner radius is applied, then the shadow is composited.
+- When `cornerRadius` or `shadow` is enabled, the output is forced to PNG (regardless of input format) to preserve transparency. Formats that support alpha (PNG, WebP, AVIF) keep their original format.
+- The shadow is shape-aware: it follows the rounded corners rather than creating a rectangular shadow.
+- Setting `borderWidth` to 0 and using only `cornerRadius` + `shadow` creates a frameless rounded shadow effect.