feat(settings): polish the import-key dialog + paste verify preview

Design pass on the Import Private/Viewing Key dialog:

- Group the optional scan-from-height section (label, slider, field, hint) in a
  subtle glass panel via a draw-list splitter so it reads as one distinct,
  clearly-optional block.
- Replace the boxed reveal / Paste / Clear buttons with frameless Material icon
  buttons: the eye toggle sits flush at the Key field's right edge (rounded to
  match the input frame), Paste / Clear are faint icon pills with tooltips.
- Add a centered %-of-chain label to the slider (mirrors the send-tab amount bar).

New: hovering Paste peeks at the clipboard and verifies it before pasting —
a tooltip classifies the clipboard the same way the field indicator does
(✓ valid type for this dialog / ⚠ a key for the other button / ✗ not a key),
with a preview line masked with '*' unless the key is currently revealed (then a
truncated peek). Empty clipboard shows "Clipboard is empty".

i18n: added paste_clip_empty (en + 8 langs); the preview reuses the field
indicator's type / wrong-type keys.

Reviewed via three adversarial multi-agent passes (splitter/panel, icon buttons,
slider math, and the paste preview's correctness/security + i18n); the confirmed
findings (eye hover-pill rounding, unrecognized-state color) are addressed here.

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

res/lang/zh.json

@@ -785,6 +785,7 @@
     "output_filename": "输出文件名：",
     "overview": "概览",
     "paste": "粘贴",
+    "paste_clip_empty": "剪贴板为空",
     "paste_from_clipboard": "从剪贴板粘贴",
     "pay_from": "付款来源",
     "payment_request": "付款请求",