Add 7 new tools and homepage with search navigation

[Rovinovic]

Summary

Adds 7 new client-side tools to the toolkit, plus a homepage and search-driven navigation to make the growing tool set easier to browse. As with every existing tool, all processing happens entirely in the browser, no data is sent to any server.

New Tools

Encode / Decode

• PEM / Certificate Decoder — Parses PEM-encoded X.509 certificates showing subject, SANs, issuer, validity, key usage, and fingerprints.
• CSR Decoder — Decodes and displays CSR fields including subject, public key, and requested extensions.

Format

• YAML Linter / Formatter — Validates, beautifies, and converts between YAML and JSON with Kubernetes-standard 2-space indentation.

Generate

• CSR Generator — Generates PKCS#10 Certificate Signing Requests with configurable subject fields and key sizes.

Utilities

• Time Zone Converter — Converts date/time across multiple time zones with quick-add presets for common regions.
• Keystore / Truststore Viewer — Reads JKS and PKCS#12 keystores entirely in-browser, displaying certificate details and chain info.
• IP / Subnet Calculator — Calculates network address, broadcast, host range, and wildcard mask from IP and CIDR or subnet mask input.

Navigation & UX

• Homepage with tool cards and a navigation refactor.
• Search bar for quickly finding tools by name.
• README updated to document the new tools.

Technical Notes

• No external API calls — all processing happens in the browser.
• No external JS libraries (vanilla JS only).
• Crypto operations use the Web Crypto API.