🎨 Palette: Add helpful placeholders to Manual Book form

[jspann21]

💡 What: Added helpful placeholder text to all form inputs within the ManualBookModal.
🎯 Why: To provide contextual guidance and examples of expected data formats (like ISBN formats, date formats, or tags) when users add a book to their library manually. This reduces friction and prevents formatting errors.
📸 Before/After: Visual changes were verified locally via Playwright screenshots, showing subtle, grey placeholder text inside empty input boxes (e.g. "e.g., 9781234567890" for ISBN-13).
♿ Accessibility: Improves cognitive accessibility by providing immediate format guidance within the input field itself, relying on native placeholder functionality.

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PR created automatically by Jules for task 14543896012416920935 started by @jspann21

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