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/res/xml/backup_rules.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Credentials must not travel to Google Drive: the pairing token grants
+     full control of the TV and the OIDC tokens are account credentials. -->
+<full-backup-content>
+    <exclude domain="sharedpref" path="pairing.xml" />
+    <exclude domain="sharedpref" path="auth.xml" />
+</full-backup-content>

app/src/main/res/xml/data_extraction_rules.xml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="utf-8"?>
+<data-extraction-rules>
+    <cloud-backup>
+        <exclude domain="sharedpref" path="pairing.xml" />
+        <exclude domain="sharedpref" path="auth.xml" />
+    </cloud-backup>
+    <device-transfer>
+        <exclude domain="sharedpref" path="pairing.xml" />
+        <exclude domain="sharedpref" path="auth.xml" />
+    </device-transfer>
+</data-extraction-rules>