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.h

@@ -302,7 +302,8 @@ public:
     void exportAllKeys(std::function<void(const std::string&, int, int)> 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,
+    // startHeight > 0 rescans from that block (shielded RPCs only; ignored for transparent WIF).
+    void importPrivateKey(const std::string& key, int startHeight,
                           std::function<void(bool, const std::string&, const std::string&)> callback);
     
     // Wallet backup
@@ -373,7 +374,8 @@ public:
     std::string screenshotDir() const;   // <config>/screenshots (fixed; sweeps overwrite in place)
 
     // Dialog triggers (used by settings page to open modal dialogs)
-    void showImportKeyDialog() { show_import_key_ = true; }
+    void showImportKeyDialog() { import_view_mode_ = false; show_import_key_ = true; }        // spending
+    void showImportViewingKeyDialog() { import_view_mode_ = true; show_import_key_ = true; }  // watch-only
     void showExportKeyDialog() { show_export_key_ = true; }
     void showBackupDialog() { show_backup_ = true; }
     void showSeedBackupDialog() { show_seed_backup_ = true; }
@@ -808,6 +810,8 @@ private:
     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)
+    bool import_view_mode_ = false;       // dialog mode: false = spending key, true = viewing key
+    char import_key_scan_height_[16] = {0};  // optional rescan start height (viewing-key mode)
     std::string backup_status_;
     bool backup_success_ = false;