[TASK]: inject polyfill via manifest.json

[sidvishnoi]

When Firefox 128 is old enough, inject directly via manifest, removing injectPolyfill from background.ts and contentScript.ts
See #606

[sidvishnoi]

We've decided to set min supported version to Firefox v140, so this is no longer blocked.

Why raise min version requirements:

• Will remove some validation warnings at publish
• Integrate permissions UI in safer manner,
• Close #607, and ease way to #1206, and
• Support Firefox Android more safely (can’t test older versions easily on Android, so supporting newer is safer).
• Current usage: v135 was the oldest version used by our users, as of May 2025. But in the last 90 days, only v145+ are used.