fix(highlight): extend prefix DiffsClear to cover quote and prefix width

Problem: in email-quoted diffs (qw > 0), the body prefix DiffsClear only
covered col 0..qw (the quote prefix). The diff prefix marker column at
col qw..pw+qw had no DiffsClear, leaving a 1-column gap where native
treesitter background could bleed through on context lines.

Solution: extend body prefix DiffsClear end_col from qw to pw + qw,
covering both the quote prefix and the diff prefix marker. On +/- lines
this was already masked by DiffsAdd/DiffsDelete at higher priority, but
context lines (prefix ' ') had no such mask.

Closes #142 issue 1.

lua/diffs/highlight.lua

@@ -480,7 +480,7 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
 
     if qw > 0 then
       pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, 0, {
-        end_col = qw,
+        end_col = pw + qw,
         hl_group = 'DiffsClear',
         priority = p.clear,
       })