fix: use relative path for fzf-lua file preview

fzf-lua's builtin previewer requires relative paths to properly locate
files for preview. Changed the field priority to use file_path (relative)
before file (absolute) to fix 'Unable to stat file' errors.

Field priority for fzf-lua entries:
1. file_path (relative) - preferred
2. path (relative)
3. filename (relative)
4. file (absolute) - fallback

Author: aweis89

lua/opencode/ui/base_picker.lua

@@ -232,7 +232,8 @@ local function fzf_ui(opts)
         -- For file preview support, append file:line:col format
         -- fzf-lua's builtin previewer automatically parses this format
         if opts.preview == 'file' and type(item) == 'table' then
-          local file_path = item.file or item.file_path or item.path or item.filename
+          -- Use relative path (file_path) for fzf-lua, not absolute (file)
+          local file_path = item.file_path or item.path or item.filename or item.file
           local line = item.line or item.lnum
           local col = item.column or item.col