Refactor hardcoded strings in MainController

[G30RG3-GJ]

Moved hardcoded strings from MainController.java to src/resources/strings.properties to improve maintainability.

• Created src/resources/strings.properties with keys for book availability messages.
• Updated MainController.java to load these strings from the resource bundle in initialize.
• Added default values for string fields to ensure safety if resource loading fails.
• Preserved existing logic for issue validation (checking against "Not Available" string) to maintain behavior parity.
• Verified resource loading functionality with a standalone test script.

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PR created automatically by Jules for task 16431855155691246102 started by @G30RG3-GJ

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