Fix TDropDown.update_value() missing UI refresh call

[Copilot]

TDropDown.update_value() did not call self.update(), so programmatic value changes (e.g. in PreferencesScreen.refresh_preferences_items()) never triggered a UI refresh — unlike update_dropdown_items() and update_error_txt() which both do.

Changes

• tuttle/app/core/views.py: Added self.update() to update_value(), wrapped in try/except RuntimeError to handle pre-mount calls gracefully (Flet 0.81 raises RuntimeError rather than returning None from .page when the control isn't yet attached)

def update_value(self, new_value: str):
    self.drop_down.value = new_value
    self.drop_down.error_text = None
    try:
        self.update()
    except RuntimeError:
        pass  # control not yet mounted

The guard is necessary because an existing test (test_update_value_before_build) covers the pre-mount path, and unconditional self.update() raises on unmounted controls in this version of Flet.

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