Refactor content scripts into platform adapter pattern

[Olawoyin007]

What

Refactor the content script architecture so each platform (Twitter, YouTube, Reddit) implements a shared adapter interface. Currently content.js is Twitter-specific.

From ROADMAP.md - Phase 3.4.

Problem

Adding YouTube and Reddit support means duplicating the classification flow, cache logic, and treatment application across multiple content scripts. This will become hard to maintain.

Proposed Architecture

extension/
  platforms/
    adapter.js        # Shared interface: extractContent(), applyTreatment()
    twitter.js        # Twitter-specific DOM selectors and extraction
    youtube.js        # YouTube-specific DOM selectors and extraction
    reddit.js         # Reddit-specific DOM selectors and extraction
  content.js          # Shared logic: observe, classify, treat (uses adapter)

Each adapter implements:

• getContentElements() - Find new content in the DOM
• extractText(element) - Pull classifiable text from an element
• applyTreatment(element, result) - Apply blur/tag/hide to an element
• getElementId(element) - Unique key for caching

Why

• Adding a new platform becomes: implement one adapter file
• Shared classification and caching logic stays DRY
• Testing is easier with a clear interface