fix: handle missing category translations gracefully

[rewritten]

Summary

• Fixes Manually creating new category breaks timebank admin #829: crash when viewing offers/needs after creating a category with a non-default locale
• Adds Category#display_name that falls back to any available translation when the current locale's name is nil or blank
• Appends [locale] to the displayed name when a fallback is used, so it's clear the name comes from another language
• Updates views and helper to use display_name instead of name directly

Test plan

• Create a category with locale=NL without filling in a name
• Switch to locale=EN and visit /offers — no crash, category appears with [nl] suffix if it has a Dutch name, or is sorted to top if truly empty
• Create a category with a name in one locale, verify it shows with [locale] indicator when viewed in a different locale

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a safer way to render category names when the current locale’s translation is missing, preventing crashes in category filter rendering/sorting.

Changes:

• Introduces Category#display_name with a fallback to any available translated name (with a locale indicator).
• Updates shared filter partials to use display_name instead of name.
• Adjusts category sorting in CategoriesHelper#all_categories to rely on to_s.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
app/models/category.rb	Adds display_name fallback logic and updates to_s to use it.
app/helpers/categories_helper.rb	Changes category sorting to use to_s for downcasing/sorting.
app/views/shared/_show_filter_hint.html.erb	Renders the current category via display_name.
app/views/shared/_post_filters.html.erb	Uses display_name in the category dropdown and items.

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