Add System Admin area and redesign Competitions page

Move the Users table and add a new rule-text editor into a dedicated
admin.html/admin.js page with its own collapsible sidebar (Users,
Rules categories), reachable only for system admins via a button next
to the brand. The Competitions page no longer renders admin-only
controls inline.

Competitions page: cards are now fully clickable to open the
competition (drop the separate "Open" button), the admin-only delete
action moves into a "..." menu (new generic openInlineMenu/
openAnchoredPopover helpers in common.js), and add an open/closed
filter.

Topbar: language switch and logout move out of the topbar into a
small popup under the profile button, which itself opens the full
settings modal for username/display name/language/password.

web/i18n/en.json

@@ -145,5 +145,16 @@
   "competition_closed": "Competition is closed",
   "upload_file": "Upload file",
   "csv_separator_hint": "Comma (,) and semicolon (;) are detected automatically.",
-  "csv_empty": "Please paste CSV or choose a file."
+  "csv_empty": "Please paste CSV or choose a file.",
+  "system_admin_area": "System Admin",
+  "user_management": "User management",
+  "rule_management": "Rule management",
+  "rules_admin_hint": "Edit the rule text, suggested penalty and escalation setting shown to scorers, per language. Saving writes directly to that language's rule file.",
+  "reload_from_disk": "Reload from disk",
+  "missing_translation": "No text yet for this language",
+  "escalation_mode_hint": "same, doubled, or escalate:tier1|tier2|…",
+  "new_rule_number": "New rule number",
+  "add_rule": "Add rule",
+  "rule_number_exists": "This rule number already exists — edit it in the table below.",
+  "missing_fields": "Please fill in all required fields"
 }