fix: Replace SHA-256 key derivation with PBKDF2

[Abfa41]

Description

Fixes #6201

This PR replaces the existing single-iteration SHA-256 key derivation with PBKDF2 to improve the security of encryption key generation.

Changes made:

• Replaced crypto.createHash('sha256') with crypto.pbkdf2Sync.
• Added a random 16-byte salt for each encryption operation.
• Stored the generated salt alongside the encrypted payload.
• Updated decryption logic to extract and reuse the stored salt.
• Configured PBKDF2 with 100,000 iterations and SHA-512.
• Continued deriving a 32-byte key for AES-256-GCM compatibility.
• Updated crypto tests to validate the new salted encrypted payload structure and expected segment count.

Security improvements:

• Introduces a computational work factor against brute-force attacks.
• Prevents identical passwords from generating identical encryption keys.
• Aligns key derivation with modern cryptographic best practices.
• Reduces risks associated with low-entropy passwords or secrets.

Pillar

• 🎨 Pillar 1 — New Theme Design
• 📐 Pillar 2 — Geometric SVG Improvement
• 🕐 Pillar 3 — Timezone Logic Optimization
• 🛠️ Other (Security improvement)

Checklist before requesting a review:

• I have read the CONTRIBUTING.md file.
• I have tested these changes locally.
• I have run npm run format and npm run lint locally and resolved all errors.
• My commits follow the Conventional Commits format.
• I have updated documentation if required.
• I have started the repo.
• I have made sure that I have only one commit to merge in this PR.

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[github-actions]

📦 Next.js Bundle Size Report (Gzipped Sizes)

✨ No significant bundle size changes detected.

📊 Summary of Totals

Category	PR Size	Base Size	Difference
Total JS	3697.00 KB	3697.00 KB	0 B
Total CSS	296.58 KB	296.58 KB	0 B

[Aamod-Dev]

Replacing the single-iteration SHA-256 hashing with crypto.pbkdf2Sync and a random salt in lib/crypto.ts significantly hardens our encryption against brute-force attacks. Updating the tests in lib/crypto.empty-fallback.test.ts to expect the 4-part payload (salt included) ensures the new structure is correctly verified. Great security improvement! Approved!

[github-actions]

🎉 Congratulations @Abfa41! Your PR has been successfully merged. 🚀

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[Abfa41]

@Aamod-Dev, please look into type labels. It should be 'type:refactor' and 'type:security' labels, not 'refactor' and 'security'. kindly change it. Thanks!