feat: add password strength indicator

[saurabhhhcodes]

What changed

• Added a live password strength indicator to the signup form.
• The meter updates as users type and labels the password as weak, medium, or strong.
• Standardized the shared auth password-toggle styles in theme.css to use CSS variables instead of hardcoded colors.

Why

• Simple passwords were easy to submit without any visual feedback.
• The shared auth styles had duplicate / hardcoded color rules that were a good fit for the theme cleanup issue.
• These small UI improvements make the auth flow safer and the stylesheet easier to maintain.

Validation

• node --check /tmp/register-inline.js
• git diff --check

Closes #393
Closes #388

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