feat: fold blocks in the large-response body viewer

[lucasAguiar11]

What & why

Collapse chevrons disappeared once a response got large. Bodies over 256 KB render in the virtualized viewer, which never had fold controls — so on a 31 MB payload the only ways to navigate were scrolling ~1.4M lines or the JSONPath filter, which answers "show me this path", not "what's in here".

This PR adds folding to the windowed viewer:

• Backend: fold_ends walks the pretty-printed body once with a bracket stack, inside the parse that already runs for windowing, and every BodyWindow carries the closing line of each line it returns. The frontend only holds a slice at a time, so it can't find a block's end itself. Unbalanced (non-JSON) text yields no folds.
• Frontend: collapsed blocks become runs of hidden lines, so rows on screen no longer map one to one onto the body. The visual ↔ real translation lives in lib/lineFolds.ts (pure, tested); useLineFolds owns the state.
• A folded row renders { … }, the way CodeMirror does — the closing line's own text when it's loaded, the mirrored bracket when it isn't.
• A search hit inside a collapsed block opens it before scrolling.
• The scrolled content moves out of VirtualBody into VirtualBodyLines to stay under the 250-line limit.

Small responses are untouched: the Rust and React entry points sit behind the same 256 KB gate (response.rs:93, BodyTab.tsx:93), so nothing on the CodeMirror path reaches a changed line. BodyWindow is the only shared type that changed, and it exists solely on the windowed path.

Closes #51

Folding individual blocks

Collapsing the whole array

How to test

1. GET https://jsonplaceholder.typicode.com/photos — 1.0 MB, past the 256 KB threshold
2. Chevrons should appear in the gutter beside every block-opening line
3. Collapse a nested object and the root array; line numbers stay real and skip the hidden range
4. ⌘F for a term inside a collapsed block — it should open and scroll to the match
5. Apply a JSONPath filter; fold state resets with the new body
6. A small response (/comments, 158 KB) still renders in CodeMirror, unchanged

Out of scope

The line-wrap toggle, which the issue mentions for the same reason. Row heights are uniform today; wrapping makes them depend on line lengths the viewer only learns as blocks load, so the scrollbar would drift on a 1.4M-line body. Worth its own change.

Checklist

• bun run typecheck, bun run lint, and the cargo checks pass (see CONTRIBUTING.md)
  • also bun run test (9 new lineFolds tests), bun run check:sizes, cargo test --workspace
• New Tauri commands are registered in collect_commands! and bun run codegen was run
  • n/a — no new commands; codegen re-run because BodyWindow gained a field
• Follows the rules in AGENTS.md (file-size limits, render-loop rules, no migration shims)