feat: add Number Base Converter tool

[Jagadeesh729]

Summary

Adds a browser-only, real-time Number Base Converter tool. It allows developers to instantly convert numbers between Decimal (base 10), Binary (base 2), Hexadecimal (base 16), and Octal (base 8) in real time with arbitrary precision using BigInt.

Closes #123

Requirements Implemented

• Input Fields: Decimal, Binary, Hexadecimal, and Octal inputs. Typing in any field instantly updates the other three.
• Validation: Shows a clear error state for invalid input (e.g. 9 in binary, G in hex) and handles incorrect characters safely without crashing.
• Arbitrary Precision: Uses native BigInt to support extremely large integers (e.g. 256-bit or 512-bit keys) without loss of precision.
• Digit Grouping: Includes a checkbox toggle to "Group digits for readability" (thousands separators for decimal, 4-bit spaces for binary, 2-character spaces for hex, 3-character spaces for octal).
• Prefix Support: Gracefully accepts base prefixes (like 0x, 0b, 0o) and handles negative numbers.
• UX & Aesthetics: Premium design matching both light and dark modes, complete with direct "Copy" buttons next to each field and sample data buttons.

Changes

• NumberBaseConverter.tsx: Logic for base conversions, state tracking, and layout design.
• number-base-converter.astro: Route path integration at /tools/number-base-converter.
• tools.ts: Tool registration in the sidebar/navigation metadata.

Verification

• Local dev server compiles and runs.
• npm run build succeeds without warnings/errors.
• Fully verified in-browser for the following test cases: decimal conversion, binary conversion, invalid input handling, digit grouping toggle, sample loaders, and clear button.

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