Normalize theme tokens: accent-text, zap-text, remove hard-coded colors

- Add accent-text and zap-text tokens to all 7 themes for proper contrast
  on accent/zap-colored buttons (fixes white-on-light-accent in Catppuccin,
  Nord Frost, Hackerman, Sepia, Gruvbox)
- Replace text-white → text-accent-text on all accent buttons (20+ instances)
- Replace text-white → text-zap-text on zap buttons
- Replace hover:text-white → hover:text-accent-text on follow/action buttons
- Replace bg-blue-500 → bg-accent in FountainCard (theme-aware)
- Remaining text-white is correct: overlays on bg-black, danger badges

src/components/thread/ThreadView.tsx

@@ -248,7 +248,7 @@ export function ThreadView() {
                   <button
                     onClick={handleRootReply}
                     disabled={!replyText.trim() || replying}
-                    className="px-2 py-0.5 text-[10px] bg-accent hover:bg-accent-hover text-white transition-colors disabled:opacity-30 disabled:cursor-not-allowed"
+                    className="px-2 py-0.5 text-[10px] bg-accent hover:bg-accent-hover text-accent-text transition-colors disabled:opacity-30 disabled:cursor-not-allowed"
                   >
                     {replySent ? "replied ✓" : replying ? "posting..." : "reply"}
                   </button>