feat: add Perplexity support

[chanyut12]

Summary

This PR adds Perplexity support to Scroll.

It introduces a new perplexity provider and wires it into the existing extension flow while keeping
the current architecture and behavior for the existing providers intact.

What Changed

• added a new Perplexity provider
• added Perplexity host matching in the extension manifest
• added Perplexity support in the existing app/provider integration points
• added Perplexity export filename normalization
• updated docs to include Perplexity in setup and testing guidance

Implementation Notes

This change was kept intentionally minimal.

• the existing provider structure was preserved
• no broad refactor was introduced
• showSidebar and the existing provider flow were kept aligned with the current codebase
• the main Perplexity-specific logic lives in its own provider file

The Perplexity parser now:

• extracts prompts from Perplexity query nodes
• extracts responses from Perplexity response containers
• pairs prompts and responses using container-aware matching and DOM-order fallbacks
• filters obvious history/sidebar noise

Validation

Ran locally:

• npm run typecheck
• npm run build
• npm run build:firefox

Manual testing completed:

• ChatGPT basic regression check passed
• Claude basic regression check passed
• Gemini basic regression check passed
• Perplexity sidebar, navigation, search, copy, and export flow passed
• Chrome and Firefox basic checks passed
• no console errors observed during core flows

Risks / Limitations

Perplexity parsing depends on the current DOM structure. If Perplexity changes its layout, the
provider may need follow-up selector updates.