Add duplicate row and insert current time via context menu

[thomaswasle]

Summary

• Add Duplicate Row to the table context menu: right-clicking any row and selecting
  "Duplicate Row" creates a new pending insertion pre-filled with all values from the source row.
  Auto-increment / serial columns are cleared so the database generates a fresh key on commit.
• Add Insert Current Timestamp to the cell context menu: right-clicking a DATE, TIME,
  DATETIME, TIMESTAMP (or equivalent) column shows a new "Insert Current Timestamp" option that
  fetches the current date/time from the database server (not the client) and writes it into
  the cell, formatted to match the column type.

Changes

Backend

• New Tauri command get_server_now(connection_id) in commands.rs — executes SELECT NOW()
  for MySQL/PostgreSQL and SELECT datetime('now', 'localtime') for SQLite, returning the raw
  timestamp string.
• Command registered in lib.rs.

Frontend

• DataGrid.tsx: duplicateSelectedRow callback copies row data (existing or pending insertion)
  and calls the new onDuplicateRow prop. setCellServerNow callback invokes get_server_now,
  parses the result with parseDateTime, and formats it via formatDateTime to match the column's
  date mode before writing through onPendingChange / onPendingInsertionChange. The "Insert
  Current Timestamp" menu item is only shown when the right-clicked column resolves to a date/time
  type.
• Editor.tsx: handleDuplicateRow creates a new pending insertion from the provided row data
  (auto-increment columns nulled out). Prop wired into <DataGrid>.
• All new menu items translated into all five supported locales (en, de, fr, es, zh).