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/data/csv-excel.md

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+---
+description: Convert between CSV and Excel (XLSX), both directions.
+---
+
+# CSV to Excel
+
+Convert between CSV and Excel (XLSX) formats in both directions. Upload a CSV or TSV file to get XLSX, or upload an XLSX file to get CSV.
+
+## API Endpoint
+
+`POST /api/v1/tools/data/csv-excel`
+
+Accepts multipart form data with a CSV, TSV, or XLSX file and a JSON `settings` field.
+
+## Parameters
+
+| Parameter | Type | Required | Default | Description |
+|-----------|------|----------|---------|-------------|
+| sheet | integer | No | `1` | Worksheet number to export when converting from XLSX (min 1) |
+
+## Example Request
+
+CSV to Excel:
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/data/csv-excel \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@data.csv" \
+  -F 'settings={"sheet": 1}'
+```
+
+Excel to CSV:
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/data/csv-excel \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@report.xlsx" \
+  -F 'settings={"sheet": 2}'
+```
+
+## Example Response
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "downloadUrl": "/api/v1/download/a1b2c3d4-e5f6-7890-abcd-ef1234567890/data.xlsx",
+  "originalSize": 2048,
+  "processedSize": 5120
+}
+```
+
+## Notes
+
+- Conversion direction is auto-detected from the input file extension: `.csv` or `.tsv` produces `.xlsx`, and `.xlsx` produces `.csv`.
+- The `sheet` parameter only applies when converting from XLSX. It selects which worksheet to export.
+- TSV (tab-separated values) files are supported alongside CSV.