fix(preview): keep cursor on match when paging with <C-d>/<C-u>

[gustav-fff]

Closes #555 (step 1 only)

Root cause

preview.scroll() in lua/fff/file_picker/preview.lua:682 set the cursor to {new_offset + 1, 0} and ran zt. The cursor (and therefore the cursorline / location highlight) tracked the viewport top instead of the match line. Pressing <C-d>/<C-u> made the highlighted line jump away from the actual match.

Fix

Page the viewport without moving the cursor. Use vim.fn.winrestview({ topline = topline }) instead of nvim_win_set_cursor + zt. Cursor stays parked on the match line set by scroll_to_line(), so the highlight stays put while content scrolls past it.

Steps to reproduce

git checkout main
make build
nvim
" :FFFFind, type a query, focus a result with location info (e.g. grep mode),
" then press <C-d>/<C-u> in the picker to scroll preview.
" Before fix: cursorline / IncSearch highlight jumps with viewport top.
" After fix: cursorline stays on the match; only viewport scrolls.

How verified

Local manual run on ~/dev/lightsource in grep mode — <C-d>/<C-u> scrolls preview viewport while highlighted match stays anchored. No change to navigation between picker results (different code path, scroll_to_line / apply_location_highlighting).

Step 2 (highlight ALL matches in file) — NOT in this PR

Out of scope for the auto-fix budget. It needs a Rust pass that scans the full file for the query (fuzzy + exact) and returns every match's byte range, then highlight_grep_matches (lua/fff/location_utils.lua:147) needs to consume that list instead of the current per-target-line fuzzy_match_ranges + ±200 line plain-text scan. @dmtrKovalenko this needs scoping — happy to take it on once the shape (Rust API + perf budget for whole-file scan during preview) is decided.

Automated triage via Gustav. Honk-Honk 🪿