fix(sessions): show fork icon + order number on nested forked sessions (#934)

* fix(sessions): fork icon + depth number, no indentation

Multiply-nested forked sessions rendered with deep indentation that crushed
the card off-screen and made titles unreadable, and the per-sibling order
number was uninformative (mostly "1").

- SessionTreeItem: replace forkIndex (sibling order) with forkDepth (nesting
  level: 1 = child of a root, 2 = fork of a fork, ...); flattenSessionTree
  populates it.
- SessionCard: render CallSplit fork icon + the depth number (primary tint).
  Removed the depth-based start padding so names stay full-width readable —
  the icon+number now carries the structure instead of indentation.
- strings: sessions_fork_indicator (en/zh/ko).
- SessionTreeTest: assert isFork/forkDepth across nested fixtures.

No backend change — backend already returns parent_session_id per SessionInfo.

* test(ws): make testAutoReconnect tolerate transient RECONNECTING state

The test gated on connectionStatus.first { it == RECONNECTING } with a
15s timeout. With the test backoff forced to 0ms the RECONNECTING state is
emitted + consumed before the collector attaches, so the wait missed it and
timed out (kotlinx.coroutines.TimeoutCancellationException) — the flake CI
saw under parallel load.

- Replace the brittle RECONNECTING .first{} wait with a tolerant check that
  accepts RECONNECTING-or-later (the reconnect clearly fired).
- Keep connect2Latch (second socket opening) as the definitive proof + add a
  final CONNECTED assertion.
- Reset the test backoff in a finally so later tests never inherit 0ms.

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Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/res/values/strings.xml

@@ -548,6 +548,7 @@
     <string name="sessions_action_pin">Pin</string>
     <string name="sessions_action_unpin">Unpin</string>
     <string name="sessions_pinned_indicator">Pinned</string>
+    <string name="sessions_fork_indicator">Forked session</string>
     <string name="sessions_rename_title">Rename Session</string>
     <string name="sessions_rename_hint">Session title</string>