feat(sourcetypes): add excel to sourcetypes files and docs about comm…

[albegosu]

Summary

• Add .xls / .xlsx upload support to the document ingestion pipeline using SheetJS (xlsx)
• Each worksheet is converted to a markdown table (## SheetName header + pipe-delimited rows) before chunking, so the RAG pipeline requires no changes
• sourceType is set to 'spreadsheet' for Excel files; the document list renders a dedicated table-cells icon for this type

Changes

Backend (server/utils/documents.service.ts)

• Import xlsx and add Excel MIMEs to ALLOWED_MIME
• isAllowedMime() — accepts .xls / .xlsx by extension as fallback
• detectFileType() — returns 'spreadsheet' for Excel inputs
• spreadsheetToText() — iterates SheetNames, converts each sheet via sheet_to_json into a markdown table
• extractTextFromUpload() — routes Excel buffers through spreadsheetToText()

Config (nuxt.config.ts)

• Default ALLOWED_FORMATS extended from pdf,md,txt → pdf,md,txt,xls,xlsx

Frontend

• pages/upload.vue — file input accept includes .xls,.xlsx
• pages/documents/index.vue — getIconForType() maps 'spreadsheet' → i-heroicons-table-cells

i18n (en.json, es.json)

• upload.supports updated to list .xls, .xlsx

Test plan

• Upload a multi-sheet .xlsx file and verify each sheet appears as a markdown table in the stored content
• Upload a legacy .xls file and confirm it is processed without errors
• Verify unsupported extensions (e.g. .docx) still return HTTP 415
• Confirm the document list shows the table-cells icon for spreadsheet documents
• Confirm PDF, TXT, and MD uploads are unaffected (no regression)
• Check that ALLOWED_FORMATS env override still works (set to pdf,txt and verify .xlsx is rejected)