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>

src/app_sweep.cpp

@@ -261,7 +261,9 @@ void App::buildSweepCatalog()
 
     // Simple bool-flag modals.
     add("modal-import-key", ui::NavPage::Overview,
-        [](App& a) { a.show_import_key_ = true; }, [](App& a) { a.show_import_key_ = false; });
+        [](App& a) { a.import_view_mode_ = false; a.show_import_key_ = true; }, [](App& a) { a.show_import_key_ = false; });
+    add("modal-import-viewkey", ui::NavPage::Overview,
+        [](App& a) { a.import_view_mode_ = true; a.show_import_key_ = true; }, [](App& a) { a.show_import_key_ = false; a.import_view_mode_ = false; });
     add("modal-export-key", ui::NavPage::Overview,
         [](App& a) { a.show_export_key_ = true; }, [](App& a) { a.show_export_key_ = false; });
     add("modal-backup", ui::NavPage::Overview,