feat(neovim): per-window winhighlight overrides (#140)

[gustav-fff]

Closes #140

Root cause

lua/fff/picker_ui.lua:961 builds one winhighlight string from hl.normal/hl.border/hl.title and applies it to all four picker windows (input, list, preview, file_info). No way to give the preview a different Normal background than prompt/list.

Fix

Add optional per-window overrides in hl: prompt_winhl, list_winhl, preview_winhl, file_info_winhl. When non-nil, the value is applied verbatim as that window's winhighlight. When nil, fallback to existing composed default. No behavior change for existing configs.

Steps to reproduce

Setup:
```lua
require('fff').setup({
hl = {
normal = 'NormalFloat',
},
})
```
Open picker. Prompt, list, and preview all share the same Normal:NormalFloat highlight. There is no exposed config option to set a different Normal highlight for the preview window only without monkey-patching nvim_open_win.

After fix:
```lua
require('fff').setup({
hl = {
list_winhl = 'Normal:NormalFloat,FloatBorder:FloatBorder,FloatTitle:Title',
prompt_winhl = 'Normal:NormalFloat,FloatBorder:FloatBorder,FloatTitle:Title',
preview_winhl = 'Normal:Normal,FloatBorder:FloatBorder,FloatTitle:Title',
},
})
```
Preview now uses regular Normal background while prompt/list use NormalFloat, matching the screenshot in the issue.

How verified

• luac -p lua/fff/conf.lua lua/fff/picker_ui.lua — parses clean.
• Manual: existing configs with only hl.normal/hl.border/hl.title still produce identical winhighlight (default branch hits the same string.format as before).
• Diff size: 15+/5-, no API breakage.

Automated triage via gustav-fff bot.