Add doctor, validate, and inspect diagnostics commands

[mintyPT]

Problem

When rendering fails, users have to infer whether the issue is Playwright installation, invalid config, missing assets, duplicate layers, missing layers, unsupported options, or output path problems.

Proposal

Add diagnostics commands:

clickclick doctor
clickclick validate ./clickclick.config.json
clickclick inspect template.html --css template.css

Acceptance criteria

• doctor checks Node version, Playwright/Chromium availability, and basic renderer startup.
• validate checks config structure, referenced templates/assets/fonts, recipes, and template sets.
• inspect reports discovered data-layer names, duplicates, fit-text elements, linked assets, and likely output dimensions.
• Diagnostics use stable warning/error codes.
• Commands support human-readable output and are compatible with future --json output.