fix(rescan): probe the node's block range off the UI thread

Finding the lowest block a node still has (Settings > Rescan, and the
automatic reconcile after a bootstrap) ran a binary search of about 42
synchronous getblockhash/getblock calls on the UI thread, each waiting
for the RPC connection behind any wallet scan (10-20 s on a big wallet)
and each with a 30 s timeout. The window froze for that long.

It now runs as one task on the wallet RPC worker, bounded to 90 s, and
the answer comes back on the UI thread; meanwhile the dialog shows
"Checking which blocks..." with Rescan disabled. An answer from an
earlier connection counts as "could not tell", and one for an earlier
opening of the dialog is dropped. The search probes block 1 first, so a
full-history node answers in one probe, and it answers "could not tell"
at once while a runtime rescan holds the lane.

Fixed along the way: any probe error (timeout, busy node) read as "block
missing", so a busy node could get a bogus "bootstrapped, rescan from
the tip"; and a non-string getblockhash reply never called back, leaving
the dialog on "Checking..." for good. Only the daemon's "can't read
block" error now counts as missing; anything else means "could not
tell".

User-visible:
- after a bootstrap, a probe that cannot tell is tried up to 6 times,
  30 s apart and only while the node is not busy, then a toast points
  to Settings > Rescan, instead of falling back to the -rescan
  restart, which fails on a bootstrapped node;
- the Settings Rescan dialog warns when the probe could not tell.

Two new strings, translated into all eight languages.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

res/lang/zh.json

@@ -69,6 +69,7 @@
     "appx_blockchain_maintenance_in_progress": "已有区块链维护操作正在进行中。",
     "appx_blockchain_rescan_complete": "区块链重新扫描完成",
     "appx_bootstrap_complete_reconciling": "引导数据完成——正在将你的钱包与新的链数据进行核对。",
+    "appx_bootstrap_reconcile_probe_failed": "无法将你的钱包与引导数据的链数据进行核对：节点一直太忙，无法检查其已有的区块。待节点同步完成后，请在设置中运行重新扫描。",
     "appx_cancel": "取消",
     "appx_cleaning_up": "正在清理…",
     "appx_confirm_your_backup": "确认你的备份",
@@ -1597,6 +1598,7 @@
     "rescan_bootstrapped_msg": "您的节点使用了引导程序，因此快照以下的区块不在磁盘上，从创世区块重新扫描将失败。请从快照所包含的高度开始重新扫描，以核对钱包的已花费余额。您的 wallet.dat 和区块链数据不会被删除。",
     "rescan_detecting": "正在检查您的节点在磁盘上已有哪些区块…",
     "rescan_from_height": "从区块高度重新扫描：",
+    "rescan_probe_failed": "无法检查您的节点在磁盘上已有哪些区块（节点正忙或请求超时）。如果此节点使用了引导程序，请取消并在一分钟后重试：在使用引导程序的节点上，下面的 -rescan 重启将失败。",
     "reset_to_defaults": "重置为默认值",
     "restarting_after_encryption": "加密后重启守护进程...",
     "restore_address": "恢复地址",