feat(settings): split import into separate key + viewing-key buttons

Backup & Data now has two distinct import actions instead of one
auto-detecting dialog:

- "Import Key" imports a spending key (transparent WIF or shielded
  z-spending key) with the strong "grants access to funds" warning.
- "Import Viewing Key" imports a watch-only shielded viewing key
  (zxviews…) with a milder eye/watch-only note and an optional
  "scan from block height" field — z_importviewingkey accepts a start
  height, so watching a recent address needn't rescan the whole chain.

The shared Material dialog (renderImportKeyDialog) branches on
import_view_mode_: title, warning tone, field label, the live type
indicator, and the recognition guard. A key valid for the *other*
button (e.g. a spending key pasted into the viewing-key dialog) now
shows a redirect hint rather than a generic "unrecognized" error.

importPrivateKey gains a startHeight arg, appended to the shielded
import RPCs (z_importviewingkey / z_importkey) and ignored for
transparent WIF (importprivkey has no start-height param). The
scan-height buffer is wiped on open/close alongside the key buffer.

i18n: 9 new keys (button label + tooltip, viewing title/note/field,
scan label/hint, two wrong-type redirect hints) with translations for
all 8 languages; CJK subset rebuilt (+1 glyph). A modal-import-viewkey
sweep surface is added for the viewing-mode dialog.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

res/lang/zh.json

@@ -441,8 +441,15 @@
     "import_key_type_zview": "屏蔽查看密钥（仅查看）",
     "import_key_warn": "只导入你自己拥有的密钥——它可访问该地址的资金。",
     "import_key_warning": "警告：切勿分享您的私钥！从不可信来源导入密钥可能会危及您的钱包安全。",
+    "import_key_wrong_type": "这看起来像查看密钥。请改用\"导入查看密钥\"。",
     "import_key_z_format": "Z 地址花费密钥 (secret-extended-key-...)",
     "import_private_key": "导入私钥...",
+    "import_viewkey_field": "查看密钥",
+    "import_viewkey_note": "仅查看：查看密钥可显示地址的余额和交易，但无法花费其资金。",
+    "import_viewkey_scan_hint": "0 = 从头重新扫描",
+    "import_viewkey_scan_label": "从区块高度开始扫描（可选）",
+    "import_viewkey_title": "导入查看密钥",
+    "import_viewkey_wrong_type": "这看起来像花费密钥。请改用\"导入密钥\"。",
     "incorrect_passphrase": "密码错误",
     "incorrect_pin": "PIN 错误",
     "insufficient_funds": "余额不足以支付此金额加手续费。",
@@ -1105,6 +1112,7 @@
     "settings_gradient_desc": "用平滑渐变替换纹理背景",
     "settings_idle_after": "之后",
     "settings_import_key": "导入密钥...",
+    "settings_import_viewkey": "导入查看密钥...",
     "settings_language_note": "注意：部分文本需要重启才能更新",
     "settings_lock_now": "立即锁定",
     "settings_locked": "已锁定",
@@ -1252,6 +1260,7 @@
     "tt_font_scale": "缩放所有文本和界面（1.0x = 默认，最大 1.5x）。",
     "tt_idle_delay": "开始挖矿前等待多长时间",
     "tt_import_key": "将私钥（zkey 或 tkey）导入此钱包",
+    "tt_import_viewkey": "导入屏蔽查看密钥以查看某个地址（只读）",
     "tt_keep_daemon": "运行设置向导时守护进程仍会停止",
     "tt_language": "钱包界面语言",
     "tt_layout_hotkey": "快捷键：左/右箭头键切换余额布局",