Add offline course viewer generator

[kovyrin]

Overview

This change adds a static offline course viewer generator on top of the existing Thinkific downloader. The new site_generator module parses the downloaded metadata, validates lesson assets, and emits an index.html two-pane experience so courses can be consumed entirely offline.

Note: this is built on top of #3 (the UI supports subtitles).

Key Features

• Build an in-memory course model that links chapters and lessons while checking local directories for required videos, HTML lessons, captions, and attachments.
• Generate responsive templates (base.html / lesson.html) plus bundled CSS and JS to mimic a Tailwind-like layout without external dependencies.
• Inline lesson content: embed <video> elements with base64-encoded captions, render text lessons, and surface downloadable attachments for each lesson.
• Provide client-side SPA behaviour for lesson switching, keyboard navigation, focus management, and URL hash syncing.
• Add a CLI entry point python -m thinkific_downloader generate-site ... with --clean, --dry-run, --downloads-dir, and --output-dir flags.

Non-Goals

• Tracking learner progress, bookmarks, analytics, or any persistent state.
• Introducing a CSS build pipeline or other external runtime dependencies.
• Serving content over HTTP; everything continues to run from the filesystem.

CLI Usage

python -m thinkific_downloader generate-site beginner-chess-mastery.json --downloads-dir downloads --clean
python -m thinkific_downloader generate-site beginner-chess-mastery.json --dry-run

Example