feat(settings): redesign the Import Key dialog

Rework the full-node Import Key dialog into a safer, Material-consistent, more
capable flow:

- Security: the key is masked by default (Password) with a reveal (eye) toggle,
  a DialogWarningHeader "only import a key you own" note, and the buffer is
  wiped with sodium_memzero on close and on successful import.
- Progress: import triggers a blocking full-chain rescan; the dialog now shows
  "Importing & rescanning — this can take several minutes" with LoadingDots and
  disables Import while it runs (no double-submit). Offline shows a "connect a
  running node" note and disables Import up front.
- Viewing keys: the classifier gains isViewingKey / isRecognizedImportKey;
  importPrivateKey auto-detects a shielded viewing key (zxviews…) and routes to
  z_importviewingkey (watch-only) vs z_importkey / importprivkey. The live type
  indicator shows Transparent / Shielded spending / Shielded viewing (watch-only).
- Result: on success the imported address is captured from the RPC result and
  shown via AddressCopyField.
- Material restyle (OverlayDialogSpec/BlurFloat + TactileButton + Esc) and full
  i18n (12 new keys x 8 languages; CJK subset rebuilt).

Verified: rendered on dark + light skins; a seeded zxviews… key shows the masked
field + "Shielded viewing key (watch-only)" indicator.

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

src/services/wallet_security_controller.h

@@ -74,9 +74,13 @@ public:
                                                 std::size_t minLength = 4);
     static KeyKind classifyAddress(const std::string& address);
     static KeyKind classifyPrivateKey(const std::string& key);
+    // True if `key` is a shielded viewing key (extended full viewing key, "zxview…" — watch-only).
+    static bool isViewingKey(const std::string& key);
     // True if `key` looks like a recognized Z (Sapling/Sprout spending) or T (WIF) private key.
-    // Single source of truth for the import dialog's indicator AND its submit guard.
     static bool isRecognizedPrivateKey(const std::string& key);
+    // As above, but also accepts a recognized shielded viewing key — the import dialog auto-detects
+    // both, so this is the single source of truth for its indicator AND its submit guard.
+    static bool isRecognizedImportKey(const std::string& key);
     static const char* importSuccessMessage(KeyKind kind);
     static std::string decryptExportFileName(std::uint64_t timestampSeconds);
     static void secureClear(std::string& value);