feat(a11y): fix SortDropdown accessibility — label, aria-label i18n, chevron

- Add sr-only <label> for the sort <select> (WCAG 1.3.1)
- Replace hardcoded German aria-label with Paraglide sort_dir_asc/desc keys
- Add custom SVG chevron overlay to restore visual dropdown indicator
  (appearance-none had removed the native browser arrow)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/en.json

@@ -201,7 +201,7 @@
 	"admin_user_delete_confirm": "Really delete user {username}?",
 	"admin_btn_new_user": "New User",
 	"admin_users_list_title": "All Users",
-	"admin_users_search_placeholder": "Search users\u2026",
+	"admin_users_search_placeholder": "Search users…",
 	"admin_users_empty": "No users found.",
 	"admin_users_select_prompt": "Select a user from the list.",
 	"admin_btn_new_group": "New Group",
@@ -464,5 +464,7 @@
 	"transcription_next_block_cta": "Mark another passage on the scan to create block {number}",
 	"transcription_draw_tooltip": "Click and drag to mark a text region",
 	"transcription_quote_stale": "Quote from an older version",
-	"transcription_block_conflict": "This block was changed by someone else — please reload"
+	"transcription_block_conflict": "This block was changed by someone else — please reload",
+	"sort_dir_asc": "Sort ascending",
+	"sort_dir_desc": "Sort descending"
 }