Add admin-configurable date display format (dd/mm/yyyy support)

[sgreadly]

Fixes the long-standing lack of dd/mm/yyyy (and other) date format support. Previously only a single generic "English" locale was available, hardcoding US-style mm/dd/yyyy-oriented display via Babel on the book detail page, and via the browser's toLocaleDateString() on the Edit Metadata page's date picker overlay.

Changes:

• Adds a Date Format field under Settings → Edit UI Configuration → Language & Display, accepting a strftime-style pattern (e.g. %d/%m/%Y, %d %b, %Y)
• Book detail page (Date Added, Published, etc.) uses this pattern directly when set
• Edit Metadata page's date picker overlay independently derives day-first vs. month-first order from the same setting (recognizing %d against %m/%b/%B), so both pages agree on ordering while keeping their own distinct visual styles (e.g. numeric 27/01/2026 in the editor vs. descriptive 27 Jan, 2026 on the detail page)
• Leaving the field blank preserves exactly the current behaviour on both pages, fully backward compatible, no migration needed, new DB column defaults to an empty string

Known edge case: if an admin sets a format containing a month token but no %d at all (unusual, unlikely in practice), the Edit Metadata page's order-detection returns unset and falls back to the original browser-locale behaviour rather than guessing.

Changelog:

• New config_date_format setting under Language & Display
• Applies to book detail page (Date Added, Published, etc.) via formatdate filter
• Edit Metadata date picker overlay independently derives day/month order from the same setting, recognising %d against %m/%b/%B
• Blank setting preserves existing behaviour exactly (backward compatible)

Closes #1525