feat: Add onboarding tour and sample rules to popup workspace

[google-labs-jules]

Why

New extension users frequently experience immediate friction and high drop-off rates because they are presented with a blank canvas and must configure complex network rules without inline examples or guidance.

To improve early-stage retention and reduce the workspace abandonment rate, we are introducing a self-contained, interactive walkthrough alongside safe, pre-configured sample rules. This onboarding experience is designed to help users understand the extension's core capabilities and successfully activate their first rule in less than a minute.

Key Decisions & Rationale

• Zero-Dependency Overlay (TourOverlay.tsx): Instead of pulling in a heavy third-party onboarding library that could bloat the extension package and cause layout shifting within the strict 540px popup width limit, we implemented a lightweight, custom React-based tour. It uses native CSS variables and a high-contrast shadow-scrim masking technique (box-shadow: 0 0 0 9999px var(--scrim)) to isolate target elements cleanly.
• Safety-First Inactive Defaults: To ensure the extension never intercepts or alters live network traffic without explicit user intent, all initialized sample rules are marked inactive (enabled: false) by default.
• Self-Contained Workspace Execution: The entire onboarding flow is housed directly within the popup workspace context, completely avoiding disruptive external tab redirects.
• Frictionless Recovery via Empty States: If users clear their rules, we prevent a dead-end experience by replacing empty panels with clear Call-to-Action (CTA) triggers to reload default sample rules or restart the onboarding tour.

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What Changed

1. Interactive Onboarding Walkthrough

• Created /app/entrypoints/popup/TourOverlay.tsx to handle tour stages, pointer alignment, and backdrop masking.
• Supports both light and dark system themes seamlessly with high-contrast, WCAG-compliant styling.
• Automatically triggers on the first launch of the popup and persists completion status (hasCompletedTour: true) to local storage to prevent unwanted repetition.

2. Self-Documenting Sample Rules

• Populated the initial state with three disabled, high-utility sample rules covering both Request and Response contexts:
  • X-Developer-Mode
  • Authorization
  • Cache-Control

3. UX Control & Empty State CTAs

• Added a "Dismiss" option on all tour steps to immediately opt-out and save state.
• Added a permanent "Tour" button in the profiles action bar to allow users to manually restart the guide at any time.
• Designed a fallback view for empty tabs featuring intuitive actions to "Reload Sample Rules" or "Restart Tour".

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Verification Checklist

• Walkthrough tour triggers automatically on the very first popup launch.
• Tour does not repeat on subsequent launches once completed or dismissed.
• Pre-populated sample rules are marked as inactive (enabled: false) by default.
• Users can dismiss the tour permanently from any step.
• Popover arrows and highlights align correctly in both light and dark system themes.
• "Reload Sample Rules" repopulates the active workspace if empty.