fix: place text field caret at end of preloaded text

[mikenye]

Fixes a TextField bug where typing into a field with an existing value could insert new characters at the start of the string instead of at the end. Fixes #50.

Problem

debugui caches textinput.Field instances per widget ID. When a TextField was reopened or reused with preloaded text, the cached field could keep an old selection/insertion point from a previous editing session.

That meant the visible text was correct, but the next typed character could still be inserted at byte 0.

A related issue existed in SetTextFieldValue, which updated the field text but reset the selection to 0,0, leaving the next insertion at the start of the string.

Fix

This change:

• resets the selection to the end of the current value whenever an unfocused text field becomes focused
• does this even when the text value already matches, because the cached field may still have stale caret state
• updates SetTextFieldValue to place the selection at the end of the new value
• reuses the same helper for other programmatic text updates in the text field code

Tests

Adds regression tests covering:

• focusing a field with preloaded text
• focusing a reused field where the text is unchanged but the cached selection is stale
• SetTextFieldValue moving the caret to the end of the new text