Fixes & polish

• The "New argument" button in the top bar now opens the create dialog from any page — no more landing on your profile with nothing happening.
• New visitors signing up are no longer shown a message about a "registration invitation" they never had; the sign-in prompt now uses plain wording.
• The Discord icon is now available in the top bar whether you're signed in or out, so the "contact us on Discord" instructions on the report page point at something you can actually see.
• Tidied up the "Argument Responses" panel: its empty-state hint now points you at the real Fork button instead of a button that didn't exist.