Add Chinese (zh_Hans) translations and fix media file access for single-machine deployment

[chinaglowing]

Summary

This PR adds 37 missing Chinese (zh_Hans) translations and fixes media file access for single-machine deployments without a reverse proxy.

Changes

Backend translations (26 entries in django.po)

• Stock status: OK ? ??
• Sales/Purchase order: Discount, discount percentage hint, line item quantity
• System settings: Edit completed transfer orders, merge purchase order line items, search results preview panel
• Error messages: Stock item no longer exists, server reloading, missing purchase/sales order permissions
• Parameter templates: Uniqueness options (none, per model type, global), uniqueness enforcement

Frontend translations (11 entries in messages.po)

• Discount, Show completed line items, Show outstanding orders, No items selected, delete confirmation text
• Direct stock, FAQ, Traceback, Filter by parts on order, render error message

Code fixes for single-machine deployment

• middleware.py: Add MEDIA_URL to paths_own_security - media files use DRF token/session auth in single-machine mode
• urls.py: Serve media files via django.views.static.serve outside DEBUG mode - fixes export 404 when no reverse proxy is configured

Testing

• Verified on Windows single-machine deployment (SQLite, port 17456, no reverse proxy)
• /api/generic/status/ returns all Chinese status labels
• Sales order line item fields display in Chinese
• Media file export (reports, labels) works without 404 errors

Notes

• Targeted at commit e4b23b4 (master branch)
• django.mo is gitignored and must be compiled separately (polib or django compilemessages)
• Frontend requires npm run compile + npm run build after applying

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[matmair]

As per our docs translations need to be submitted on crowdin

[chinaglowing]

Thanks for the pointer! I've split this PR:\n\n- Translations (37 entries): Will be submitted via Crowdin as per project guidelines\n- Code fix (media file access): Moved to #12732 which contains only the middleware/urls changes without any translation files