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/app.h

@@ -300,7 +300,10 @@ public:
     void exportPrivateKey(const std::string& address, std::function<void(const std::string&)> callback);
     // callback receives (keys, exportedCount, totalAddresses) so callers can detect a keyless/partial export.
     void exportAllKeys(std::function<void(const std::string&, int, int)> callback);
-    void importPrivateKey(const std::string& key, std::function<void(bool, const std::string&)> callback);
+    // callback(success, errorOrEmpty, importedAddress). address is "" on failure or when the RPC
+    // returns none; the import routes to z_importviewingkey / z_importkey / importprivkey by key type.
+    void importPrivateKey(const std::string& key,
+                          std::function<void(bool, const std::string&, const std::string&)> callback);
     
     // Wallet backup
     void backupWallet(const std::string& destination, std::function<void(bool, const std::string&)> callback);
@@ -802,6 +805,9 @@ private:
     std::string export_address_;
     std::string import_status_;
     bool import_success_ = false;
+    bool import_key_reveal_ = false;      // show the key in plaintext (default masked)
+    bool import_in_progress_ = false;     // an import + rescan is running (disable/spinner)
+    std::string import_result_address_;   // address imported on success (shown as a copy field)
     std::string backup_status_;
     bool backup_success_ = false;