fix(security): throttle and wipe the import dialog clipboard preview

Hovering the Paste button in the import-key dialog read the OS clipboard
every frame to draw a ghost preview of the key, and the trimmed
plaintext was rebuilt each frame and never wiped. SDL's clipboard read
is a synchronous round-trip, and the pasted value is a secret private or
viewing key.

- The clipboard is now read only on hover entry and then at most twice a
  second, and the classified preview is cached rather than recomputed
  every frame.
- The cached preview is a dedicated secret buffer: it is wiped the moment
  it stops being shown, wiped on secret wipe, and the transient trimmed
  copy is zeroed right after it is classified.
- Trimming moves to a shared clipboard helper shared with the paste path.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app.h

@@ -1499,6 +1499,10 @@ private:
     bool export_in_progress_ = false; // async key fetch running (spinner + disable Export)
     bool export_error_ = false;       // last export returned no key (locked wallet / failure)
     char import_key_input_[512] = {0};
+    char    import_clip_preview_[64] = {0};   // trimmed clipboard preview (<=48 chars + NUL); SECRET, wiped
+    uint8_t import_clip_class_       = 0;      // 0 none / 1 ok / 2 wrong-type / 3 unrecognized
+    double  import_clip_next_at_     = 0.0;    // ImGui::GetTime() of next allowed clipboard read
+    bool    import_clip_hover_prev_  = false;  // hover edge
     std::string export_address_;
     std::string import_status_;
     bool import_success_ = false;