fix: use UTF-16 code units for description link offsets

YouTube provides startIndex and length values in UTF-16 code units,
but the code was iterating Unicode codepoints. This mismatch caused
links to be misaligned when the description contained emoji or other
supplementary characters before a link.

Fixes #5821

src/invidious/videos/description.cr

@@ -1,76 +0,0 @@
-require "json"
-require "uri"
-
-private def copy_string(str : String::Builder, iter : Iterator, count : Int) : Int
-  copied = 0
-  while copied < count
-    cp = iter.next
-    break if cp.is_a?(Iterator::Stop)
-
-    if cp == 0x26 # Ampersand (&)
-      str << "&amp;"
-    elsif cp == 0x27 # Single quote (')
-      str << "&#39;"
-    elsif cp == 0x22 # Double quote (")
-      str << "&quot;"
-    elsif cp == 0x3C # Less-than (<)
-      str << "&lt;"
-    elsif cp == 0x3E # Greater than (>)
-      str << "&gt;"
-    else
-      str << cp.chr
-    end
-
-    copied += 1
-  end
-
-  return copied
-end
-
-def parse_description(desc, video_id : String) : String?
-  return "" if desc.nil?
-
-  content = desc["content"].as_s
-  return "" if content.empty?
-
-  commands = desc["commandRuns"]?.try &.as_a
-  if commands.nil?
-    # Slightly faster than HTML.escape, as we're only doing one pass on
-    # the string instead of five for the standard library
-    return String.build do |str|
-      copy_string(str, content.each_codepoint, content.size)
-    end
-  end
-
-  iter = content.each_codepoint
-
-  index = 0
-
-  return String.build do |str|
-    commands.each do |command|
-      cmd_start = command["startIndex"].as_i
-      cmd_length = command["length"].as_i
-
-      # Copy the text chunk between this command and the previous if needed.
-      length = cmd_start - index
-      index += copy_string(str, iter, length)
-
-      # We need to copy the command's text using the iterator
-      # and the special function defined above.
-      cmd_content = String.build(cmd_length) do |str2|
-        copy_string(str2, iter, cmd_length)
-      end
-
-      link = cmd_content
-      if on_tap = command.dig?("onTap", "innertubeCommand")
-        link = parse_link_endpoint(on_tap, cmd_content, video_id)
-      end
-      str << link
-      index += cmd_length
-    end
-
-    # Copy the end of the string (past the last command).
-    remaining_length = content.size - index
-    copy_string(str, iter, remaining_length) if remaining_length > 0
-  end
-end