refactor(rules): split ui-ux.md into index + references

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

shared/rules/ui-ux/references/forms.md

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+## Forms
+
+* Keep forms as short as practical.
+* Ask only for information that is actually required.
+* Group related fields together.
+* Use clear field labels.
+* Do not rely on placeholders as the only label.
+* Use appropriate input controls for the expected value.
+* Provide useful defaults where safe and predictable.
+* Preserve user input when validation or network errors occur.
+* Validate fields at useful moments without interrupting normal input.
+* Show validation errors close to the affected field.
+* Explain how to fix invalid input.
+* Avoid technical validation messages.
+* Clearly mark optional fields where useful.
+* Avoid asking the same information multiple times.
+* Use autocomplete and autofill where appropriate.
+* Make keyboard navigation logical.
+* Use sensible tab order.
+* Support Enter and Escape behavior where users expect it.