fix: add cwd to fzf-lua config for proper path resolution

fzf-lua's builtin previewer uses opts.cwd to resolve relative paths.
Without this, the previewer cannot find files and shows 'Unable to stat
file' errors even though the paths are correct.

From fzf-lua's entry_to_file():
  if cwd and #cwd > 0 and not isURI and not M.is_absolute(stripped) then
    stripped = M.join({ cwd, stripped })
  end

This fix ensures relative paths like 'lua/opencode/ui/highlight.lua:37:'
are properly resolved by prepending the cwd.

Author: aweis89

lua/opencode/ui/base_picker.lua

@@ -211,6 +211,8 @@ local function fzf_ui(opts)
       _headers = { 'actions' },
       -- Enable builtin previewer for file preview support
       previewer = opts.preview == 'file' and 'builtin' or nil,
+      -- Set cwd for fzf-lua to resolve relative paths in preview
+      cwd = vim.fn.getcwd(),
       fn_fzf_index = function(line)
         -- Strip the appended file:line:col info before matching
         local display_part = line:match('^([^\t]+)') or line