Make existing controls real + add robustness pass

Wire up controls that were rendered but inert, expose unused server
actions, and harden the live update path.

Controls now functional:
- File priority dropdowns (Content tab) now persist via new
  setFilePriority server action
- Context menu: Set location, Limit download/upload rate, Set share
  limit (wire setSavePath/setDownLimit/setUpLimit/setRatioLimit)
- Trackers sidebar filter actually filters (snapshot now carries
  per-torrent trackerHosts)

Robustness:
- Grid updates cells in place when row order/columns are unchanged
  (preserves text selection/scroll, no per-second listener re-bind)
- Boot retries until the server responds; SSE drops show a
  "Reconnecting" banner; failed actions/add/delete surface a toast
- Prune selection/detail for torrents that disappear
- Drop Backspace-as-delete (kept Delete)

Cleanup:
- Remove pseudo DHT/PeX/LSD names from the real-tracker host pool in
  mock data (they leaked into announce URLs and the sidebar)
- Remove dead detailTab import/export

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/index.js

@@ -68,10 +68,18 @@ function snapshot() {
       forceStart: t.forceStart,
       timeActive: t.timeActive,
       private: t.private,
+      trackerHosts: trackerHostsOf(t),
     })),
   };
 }
 
+function hostOf(url) {
+  try { return new URL(url).host; } catch { return url; }
+}
+function trackerHostsOf(t) {
+  return t.trackers.filter((tr) => tr.tier >= 0).map((tr) => hostOf(tr.url));
+}
+
 function json(res, data, code = 200) {
   const body = JSON.stringify(data);
   res.writeHead(code, { 'Content-Type': 'application/json; charset=utf-8' });
@@ -107,6 +115,7 @@ const ACTIONS = {
   setDownLimit: (t, p) => { t.downLimit = p.limit ?? 0; },
   setUpLimit: (t, p) => { t.upLimit = p.limit ?? 0; },
   setRatioLimit: (t, p) => { t.ratioLimit = p.limit ?? -1; },
+  setFilePriority: (t, p) => { if (t.files[p.index]) t.files[p.index].priority = p.priority; },
   topPriority: (t) => { t.priority = 1; },
   bottomPriority: (t) => { t.priority = 99; },
   increasePriority: (t) => { t.priority = Math.max(1, (t.priority || 1) - 1); },