Resolve the full review backlog (issues #1-#13)

Control server security and robustness:
- Socket read timeout, handshake deadline and a client cap so an idle or
  hostile connection can no longer pin threads forever (#1)
- Per-address rate limiting that counts every failed hello, Origin
  checking on the upgrade, and a separate revocable session token for
  code-authenticated clients so the guessable path no longer yields the
  QR credential (#2)
- Credentials excluded from cloud backup and device transfer, constant
  time comparisons, and a pairing reset in the settings (#3)
- Playlist fetches restricted to http(s), capped at 24 MB and bounded by
  an overall transfer deadline (#4)
- Port conflicts and MediaSession id collisions no longer crash the app;
  the remote degrades to unavailable with a plain-language note (#11)

Player:
- Seeking no longer collapses to position 0 when the duration is unknown
  (#5)
- Pause acts on playWhenReady, so pausing during a stall works and
  playback cannot resume in the background after leaving the app (#6)
- Playback failures stay on screen with a retry action instead of
  silently dropping back to the list (#7)
- Reconnects are spaced 1s/3s/8s and re-entering the channel just closed
  waits out a short grace period, which is what the provider needs to
  release the previous session (#12)

Channel list and remote:
- Leaving playback returns to the channel the viewer came from (#13)
- The remote only rebuilds its list when the data changed, never
  overwrites a focused input and carries indices instead of scanning (#8)
- Pairing retry reconnects properly, resets its backoff and validates the
  code before spending an attempt (#9)
- M3U parsing keeps commas in names, strips a BOM and rejects payloads
  that are not playlists, covered by unit tests under tests/ (#10)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/src/main/assets/remote/index.html

@@ -320,6 +320,7 @@
     ws.onclose = () => {
       state.connected = false; state.authorized = false;
       renderConnection();
+      state.ws = null;
       setTimeout(connect, state.retryDelay);
       state.retryDelay = Math.min(state.retryDelay * 1.6, 10000);
     };
@@ -340,7 +341,7 @@
         if (msg.token) localStorage.setItem('castarr_token', msg.token);
         $('tv-name').textContent = msg.device || 'TV';
         if (msg.status) { state.status = msg.status; }
-        if (msg.channels) { state.channels = msg.channels; }
+        if (msg.channels) { state.channels = msg.channels.map((c, i) => { c._idx = i; return c; }); }
         if (msg.extras) { state.extras = msg.extras; }
         state.playlistUrl = msg.playlistUrl || '';
         $('pair-error').textContent = '';
@@ -358,10 +359,10 @@
       case 'status':
         state.status = msg;
         renderStatus();
-        renderChannels();
+        renderChannelsIfChanged();
         break;
       case 'channels':
-        state.channels = msg.channels || [];
+        state.channels = (msg.channels || []).map((c, i) => { c._idx = i; return c; });
         state.playlistUrl = msg.playlistUrl || '';
         if (msg.extras) { state.extras = msg.extras; }
         renderChannels();
@@ -446,6 +447,7 @@
       $('setup').classList.add('open');
       return;
     }
+    list.innerHTML = '';
     const favSet = new Set(state.extras.favorites || []);
     const favOn = state.extras.favoritesSupported && state.favOnly;
     const shown = favOn ? filtered.filter((c) => favSet.has(c.backendId)) : filtered;
@@ -462,9 +464,18 @@
       });
       list.appendChild(bar);
     }
+    if (!shown.length) {
+      const empty = document.createElement('div');
+      empty.className = 'empty';
+      empty.textContent = favOn
+        ? 'Keine Favoriten — Stern auf einem Sender antippen.'
+        : 'Keine Sender gefunden.';
+      list.appendChild(empty);
+      return;
+    }
     const frag = document.createDocumentFragment();
     shown.slice(0, 500).forEach((c) => {
-      const idx = state.channels.indexOf(c);
+      const idx = c._idx;
       const btn = document.createElement('button');
       btn.className = 'chan' + (c.url === state.status.url || c.name === state.status.channel ? ' current' : '');
       const num = document.createElement('span');
@@ -517,12 +528,27 @@
       btn.addEventListener('click', () => playChannel(c));
       frag.appendChild(btn);
     });
-    list.innerHTML = '';
     list.appendChild(frag);
-    if (state.playlistUrl) $('playlist-input').value = state.playlistUrl;
+    // Never overwrite an input the user is typing in.
+    const pl = $('playlist-input');
+    if (state.playlistUrl && document.activeElement !== pl) pl.value = state.playlistUrl;
   }
 
-  function renderAll() { renderStatus(); renderChannels(); }
+  // Rebuilding the list on every 2s status push swallowed taps that landed
+  // between touchstart and touchend. Only rebuild when something changed.
+  let listSignature = '';
+  function renderChannelsIfChanged() {
+    const sig = [
+      state.channels.length, state.searchTerm, state.favOnly,
+      (state.extras.favorites || []).join(','),
+      state.status.channel || '',
+    ].join('|');
+    if (sig === listSignature) return;
+    listSignature = sig;
+    renderChannels();
+  }
+
+  function renderAll() { renderStatus(); listSignature = ''; renderChannels(); }
 
   // --- actions ---
   function playChannel(c) {
@@ -583,10 +609,17 @@
 
   $('pair-btn').addEventListener('click', () => {
     const code = $('code-input').value.trim();
-    if (code.length !== 4) { $('pair-error').textContent = 'Bitte 4 Ziffern eingeben.'; return; }
+    if (!/^[0-9]{4}$/.test(code)) {
+      $('pair-error').textContent = 'Bitte 4 Ziffern eingeben.';
+      return;
+    }
+    // A fresh attempt must not inherit the backoff from earlier failures.
+    state.retryDelay = 1000;
+    $('pair-error').textContent = 'Verbinde…';
     manualCode = code;
     $('pair-error').textContent = '';
-    if (state.ws) state.ws.close(); else connect();
+    // A closed socket ignores close(), so the retry has to reconnect.
+    if (state.ws && state.ws.readyState <= 1) state.ws.close(); else connect();
   });
 
   let toastTimer = null;