refactor(ui/details): single-allocation hex render for STUN Transaction ID

[Pablosinyores]

Summary

Detail panel rendered the STUN Transaction-ID display string with

info.transaction_id.iter().map(|b| format!("{:02x}", b)).collect::<String>()

which yields 13 heap allocations per render: one short `String` per byte across the 12-byte RFC 5389 transaction ID, plus the final `String` into which they are collected (`src/ui/tabs/details.rs:867`).

Rewrite to a single `String::with_capacity(bytes.len() * 2)` + `write!` per byte:

• One allocation, sized exactly (2 hex chars per byte = 24).
• `write!`-to-`String` is infallible; `Result` discarded via `let _`.
• Same observable hex output (lowercase, zero-padded, no separator).

Why this shape

Matches the merged single-alloc hex pattern from

575ed75 refactor(dpi/bittorrent): single-allocation hex_encode for info-hash render (#307)

applied to the parallel UI render path. `info.transaction_id` is `[u8; 12]` so the capacity hint is exact.

Test plan

• `cargo fmt` clean
• `cargo clippy --all-targets -- -D warnings` clean
• `cargo test --lib` 365/365 (including `ui::snapshot_tests::*`)

No behavior, layout, or copy change.

[domcyrus]

Hi @Pablosinyores, thanks for the PR and welcome to RustNet!

The change is correct and clean: the capacity hint is exact, the import is right, and the output is identical to before.

Two pointers from CONTRIBUTING.md on how we weigh changes like this:

1. Performance & Optimizations. A perf-motivated PR should show the code is on a real hot path, ideally a flamegraph under real traffic (see PROFILING.md). The detail panel render only runs for the single selected connection while a STUN connection is open, so it is a cold path. The guideline notes these micro-optimizations may be closed with thanks, since the gain is not meaningful.

2. Issue first for refactors. Non-trivial refactors should start with an issue so we can sanity-check fit before you invest time. (Typo, docs, and small bug fixes are exempt.)

None of this is a knock on the code. If you want to dig into allocation work, profiling with PROFILING.md to find a genuinely hot path is the best place to start. Thanks again, and please keep them coming!