🎨 Palette: Form label and modal focus improvements

[jspann21]

💡 What: Added autoFocus to the primary input in ManualBookModal, converted a heading to a semantic <label> in IntegrationsSection, and added an aria-label to the folder input in LibrarySourcesSection.
🎯 Why: To improve keyboard navigation flow when opening modals, and to ensure screen readers can properly announce form inputs in settings.
♿ Accessibility: Screen reader users will now hear the correct label associations for the API key and folder inputs. Keyboard users will immediately have focus on the first interactive element when opening the Add Book modal without needing extra tab presses.

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PR created automatically by Jules for task 5713560957112697567 started by @jspann21

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