feat: Add FCM analytics label support

[jacog2]

Summary

• Support analytics_label in converted FCM push payloads.
• Validate labels against Firebase's allowed 1-50 character pattern before mapping to fcmOptions.analyticsLabel.
• Keep analytics metadata out of the client-visible converted Android/APNS payload data and document usage.

Closes #377.

Validation

• npm ci completed with existing engine warnings because this machine has Node v25.9.0 while the package supports Node 20/22/24.
• $env:TESTING='1'; .\node_modules\.bin\jasmine.cmd spec\FCM.spec.js
• $env:TESTING='1'; .\node_modules\.bin\jasmine.cmd
• .\node_modules\.bin\eslint.cmd src\FCM.js spec\FCM.spec.js
• git diff --check

Note: npm run lint was not usable on this Windows checkout because the repository-wide ESLint linebreak-style rule flags untouched CRLF files; the touched JS files pass ESLint directly.

[parse-github-assistant]

I will reformat the title to use the proper commit message syntax.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 6d96a138-ce33-4380-ad6a-8299478bff14

📥 Commits

Reviewing files that changed from the base of the PR and between cad9b14 and aa98ecf.

📒 Files selected for processing (3)

• README.md
• spec/FCM.spec.js
• src/FCM.js

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🧪 Generate unit tests (beta)

• Create PR with unit tests

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[jacog2]

Ready for review. I kept the PR focused on issue #377 and validated the full Jasmine suite plus ESLint on the touched JS files.