Construct ASCII arches

Python 2, 106 bytes (94 chars)

n=input();j=1
exec"s=j/2*'║';print s+'╔'+'═'*(2*n-j)+'╗'+s;j+=2;"*n
if n:t='╨'*n;print t+'─'+t

Pretty straightforward. Prints line by line with a changing number of horizontal and vertical bars. The last line is printed separately.

I feel like I'm missing some optimizations. The fact that the chars are multiple bytes means you can't do something like '║╨'[n>0], so I didn't find a good way to print the last line in the loop. It's ugly that there's so much manipulation going on with the counter. I'd like update strings directly, like s+='║', but the index is also used for horizontal bars.

Perl, 78 82 chars

$n='─';$_='══'x pop;while(s/══//){print"$s╔═$_╗$s\n";$s.="║";$n="╨$n╨"}$s&&print$n

Sadly, I couldn't figure out a way to take advantage of the bonus without increasing the size by more than 10%. I may yet prevail.

Ungolfed

Pretty straightforward, really. Builds up bottom line (╨$n╨) incrementally, while shortening top line (══) by two characters, ending when it can no longer be shortened, so I don't have to mess with counters.

 $n = '─'; # Bottom line
 $_ = '══'x pop; # "Top" line, length from commandline argument
 while (s/══//) { # Shorten top line by two characters
     print "$s╔═$_╗$s\n"; # Print current line with $s (sides)
     $s .= "║";           # Append vertical bar to sides
     $n  = "╨$n╨";        # Widen bottom line
 }
 $s && print $n; # Print bottom line if input is not 0