fix(columns): hide misleading directory sizes in size column

Problem: the size column shows the filesystem inode size (typically
4096 = 4.1k) for directories, which is misleading — users expect no
size for directories.

Solution: add an early return for directory entries in the size render
function of the files, SSH, and S3 adapters.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lua/canola/adapters/files.lua

@@ -60,6 +60,9 @@ file_columns.size = {
     if not stat then
       return columns.EMPTY
     end
+    if entry[FIELD_TYPE] == 'directory' then
+      return columns.EMPTY
+    end
     if stat.size >= 1e9 then
       return string.format('%.1fG', stat.size / 1e9)
     elseif stat.size >= 1e6 then