Add manual channel marker feature on YouTube home page

[tri-pti]

This PR introduces a lightweight feature that allows users to manually mark YouTube channels directly on the home page.

A small star icon (☆ / ⭐) appears next to channel names. Clicking the star toggles the marked state, which persists across sessions.

Motivation

YouTube does not reliably expose subscription information on the home page due to client-side rendering, A/B testing, and dynamic hydration. Multiple automatic approaches were explored, but all were inconsistent.

This PR provides a user-controlled and stable alternative that avoids fragile scraping or internal APIs while solving the core UX problem.

Features

• Click-to-mark channels using a star icon
• Persistent storage of marked channels
• Prevents accidental navigation when clicking the star
• Works with infinite scrolling on the home page
• No API usage or additional permissions

Implementation Notes

• Implemented as an isolated feature (channel-marker.js)
• Integrated into the existing feature lifecycle
• Uses existing storage mechanisms
• Minimal DOM injection, scoped to channel links only

Design Decisions

• Manual marking is intentional to ensure long-term reliability
• No attempt to infer subscription status
• No background scripts or external requests

Checklist

• Feature works on YouTube Home
• No navigation triggered on star click
• Persistent state verified
• No console errors
• Minimal and isolated changes

[ImprovedTube]

thanks! #3552 (comment)