Hide form placeholders when submit is focused

[GreyforgeLabs]

Summary

• Show empty field placeholders only while the input field itself is focused
• Leave empty fields visually blank when focus moves to the submit button
• Add render-level regression tests for focused field and submit-focused states

Validation

• git diff --check
• cargo test -p cortex-tui widgets::form::tests:: --no-default-features
• cargo check -p cortex-tui --no-default-features

Validation blocker

• cargo fmt --package cortex-tui --check reports an unrelated existing formatting diff in src/cortex-tui/src/question.rs; this PR leaves that file untouched to keep scope narrow.

Addresses PlatformNetwork/bounty-challenge#53129.

[greptile-apps]

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