Update assistant package

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Summary

Migrate rpaframework-assistant from robocorp-flet==0.4.2.3 (fork of flet 0.4.2) to official flet>=0.82.0,<1.0.0.

Breaking changes

• Python minimum version bumped from >=3.9.1 to >=3.10 (flet 0.82+ requirement)

Core changes

• background_flet.py — Full rewrite. Replaced removed flet internals (__connect_internal_sync, open_flet_view, close_flet_view) with
  ft.app() in a daemon thread, threading.Event for close detection, and signal.signal patching. Added 30-second startup timeout.
• flet_client.py — Window API migration (page.window_height → page.window.height etc.), thread-safe page mutations via
  _run_on_flet_thread(), FilePicker service registration via page._services.register_service(), Dropdown on_change → on_select,
  AppBar.actions null-safety.
• library.py — Import paths (flet_core → flet), async FilePicker (await pick_files()), async page.launch_url(), Icon enum conversion
  for add_flet_icon, state cleanup between dialogs (date_inputs, _required_fields, _open_layouting, validation_errors), empty date input
  returns None instead of ValueError.
• callback_runner.py — Import path update only.
• types.py — Import path update, alignment constants replaced with explicit Alignment(x, y) values.

Bug fixes found during testing

• Button click then submit caused timeout — page.update() from wrong thread corrupted flet state
• Icons rendered as blank white area — string names must be converted to Icons enum
• FilePicker crashed with "'coroutine' object is not iterable" — pick_files() is now async
• FilePicker error "Control must be added to page first" — page.services.append() doesn't mount controls
• State leaked between sequential dialogs — date_inputs, _required_fields, _open_layouting not cleared
• Empty date field crashed with ValueError: Invalid isoformat string: ''
• Link click warning about unawaited coroutine — page.launch_url() is now async

Tests

• Added tests/robot/test_elements.robot with 31 manual test cases covering all UI keywords with Pass/Fail buttons for tester verdict

Docs

• Added docs/Change-to-flet.md with full migration reference