feat(webapp): storybook gallery for the dashboard agent

[kathiekiwi]

Split out of #4418 so that PR carries only what ships to users.

Adds the agent gallery: five storybook.agent-* pages (chat UI, view blocks, report, investigation, watch) with their shared shell and manifest, the demo fixtures and two demo-only cards that feed them, the agent toast examples, and the screenshot script.

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Latest commit: 662e92f

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[coderabbitai]

Walkthrough

Added client-safe dashboard-agent demo fixtures for identifiers, charts, view blocks, investigations, intents, watches, and messages. Added reusable chart and intent components. Replaced the diagnosis-only Storybook route with a gallery framework and routes for agent UI, view blocks, reports, investigations, and watches. Added toast examples, gallery navigation, theme controls, validation tests, and a Playwright screenshot utility with optional chat capture support.

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 6.15% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.
Description check	⚠️ Warning	The description explains the main changes and includes a screenshot, but it omits the required issue, checklist, testing, and changelog sections.	Add the required template sections, complete the checklist, document testing steps, provide a changelog entry, and include the issue reference.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly summarizes the primary change: adding a Storybook gallery for the dashboard agent.

✨ Finishing Touches 💡 1

📝 Generate docstrings 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/agent-storybook-gallery

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[devin-ai-integration]

Devin Review found 1 new potential issue.