fix(preview): highlight all matches per file in grep preview

[gustav-fff]

Closes #555 (step 2). Step 1 already shipped in #556.

Root cause

location_utils.highlight_grep_matches only highlighted the cursor's match line. In fuzzy mode it explicitly returned after that line; in plain/regex it scanned ±200 lines via string.find, which still missed any match the cursor wasn't parked on (no per-line range visibility).

Fix

Plumb every loaded match for the previewed file from picker_ui through location.file_matches = { {line, ranges}, ... }, then highlight each range using ripgrep-supplied byte offsets (match_byte_offsets in Rust → match_ranges in Lua). Same path covers plain, regex, and fuzzy — all three populate the offsets in crates/fff-core/src/grep.rs:272. Falls back to the existing literal scan when file_matches is absent.

lua/fff/picker_ui.lua:1443 collects matches sharing relative_path. lua/fff/location_utils.lua:152 consumes them.

Steps to reproduce

git clone https://github.com/dmtrKovalenko/fff.git && cd fff
git checkout main
make build
nvim -c 'lua require("fff").setup()' some_repo_with_many_hits/

Then :FFFFind, switch to grep mode, type a query that hits the same file on multiple lines (e.g. function).

• Pre-fix: only the line under the cursor is highlighted in the preview.
• Post-fix: every matching line in the file is highlighted simultaneously, with exact byte spans.

How verified

make lint (rust). Lua linter is broken locally (missing /opt/homebrew/opt/lua/bin/lua5.4); change is mechanical Lua and structurally mirrors the existing fuzzy path that already shipped in #556.

Automated triage via Gustav. Honk-Honk 🪿