Update color definitions and spelling options

Refactor diagnostic colors for better visibility in
editor and plugin highlight groups. Add new spelling
options for enhanced functionality.

lua/paper-tonic-modern/groups/syntax.lua

@@ -76,8 +76,8 @@ return {
   
   Underlined = { underline = true },
   Ignore = { fg = c.fg_weak },
-  Error = { fg = c.alert_strong, bold = true },
-  Todo = { fg = c.alert_strong },
+  Error = { fg = c.diag_error, bold = true },
+  Todo = { fg = c.diag_error },
   
   -- ============================================================================
   -- Language-Specific: CSS