refactor(network): look up LISH search detail row by id via $derived

[lukyrys]

Summary

Followup refactor to PR #22. Replaces the in-place row mutation in lishSearch.handleUpdate with a clean replace-by-id pattern, and moves the LISH peer-list detail page from holding a snapshot reference to looking up the row reactively by id.

Why

PR #22's frontend change kept the LishSearchResult row identity stable across updates because Network.svelte held a direct reference to the row object (lishPeerListRow = row). That worked but introduced two cognitive-load issues flagged in code review:

1. Mixed reactivity model — new LISH = new reference, update of existing = in-place mutation. Future contributors have to remember which path applies when.
2. Snapshot reference is fragile — any future change to the search reducer that replaces row objects would silently break the open detail page again.

What this PR does

Network.svelte — hold the LISH id, derive the row:

let lishPeerListRowID = $state<string | null>(null);
let lishPeerListRow = $derived<LishSearchResult | null>(
    lishPeerListRowID === null ? null : search.results.find(r => r.id === lishPeerListRowID) ?? null
);

lishSearch.svelte.ts handleUpdate — simplified back to replace-by-id:

const byID = new Map(results.map(r => [r.id, r] as const));
for (const row of d.lishs) byID.set(row.id, row);
results = [...byID.values()];

The search reducer is no longer responsible for preserving object identity — consumers that need cross-update tracking look up by id.

Edge case

When search.clear() runs (or the row disappears from results), $derived returns null and the {:else if ... && lishPeerListRow} guard unmounts the detail page. This matches the search session lifecycle (max network.searchTimeout, default 30 s) — showing a stale frozen snapshot of a finished search would arguably be the worse UX.

Dependency

Stacks on top of PR #22 (4 commits behind in this branch). Merge order: #22 first, then this. If #22 is rebased / amended, this branch needs to follow.

Test plan

• bun run check (frontend) — 0 errors, 0 warnings
• Manual: open search, click LISH detail, verify peer count updates as new responders arrive
• Manual: close search while detail is open — detail unmounts cleanly