feat: enable vim syntax fallback by default (#152)

## Problem

Languages without a treesitter parser (COBOL, Fortran, etc.) got no
syntax highlighting because \`highlights.vim.enabled\` defaulted to
\`false\`.

## Solution

Flip the default to \`true\`. The vim syntax path is already deferred
via \`vim.schedule\` so it never blocks the first paint. \`max_lines =
200\` stays unchanged — appropriate given the ~30x slower per-hunk cost
vs treesitter.

lua/diffs/init.lua

@@ -124,7 +124,7 @@ local default_config = {
       max_lines = 500,
     },
     vim = {
-      enabled = false,
+      enabled = true,
       max_lines = 200,
     },
     intra = {