refactor: process picker themed styling, context menu, auto-select

Extract shared init into initUi(). Apply dark theme styling from global
palette to table, header, filter, and buttons. Add right-click context
menu with Copy PID/Name/Path. Auto-select last attached process on open.
Remove duplicate attach->accept() connection from .ui (handled in code).

src/processpicker.h

@@ -35,9 +35,11 @@ private slots:
     void filterProcesses(const QString& text);
 
 private:
+    void initUi();
     void enumerateProcesses();
     void populateTable(const QList<ProcessInfo>& processes);
     void applyFilter();
+    void selectPreferredProcess();
 
     Ui::ProcessPicker *ui;
     uint32_t m_selectedPid = 0;