feat: add dark mode theme with toggle

[devin-ai-integration]

Summary

Adds a full dark mode theme to the Conduit application with a toggle button in the navbar.

What's included:

• ThemeService — manages theme state using Angular signals, persists preference to localStorage, and respects prefers-color-scheme on first visit
• ThemeToggleComponent — moon/sun toggle button in the header navbar, accessible with aria-label
• dark-theme.css — comprehensive CSS variable overrides scoped to [data-theme="dark"] covering all UI elements: navbar, cards, forms, pagination, tags, banners, article pages, profile pages, footer, sidebar, and buttons

The implementation leverages the existing CSS custom property system in styles.css, overriding variables under the [data-theme="dark"] selector without modifying the shared submodule theme file.

Screenshots

Light mode (default):

Dark mode:

Review & Testing Checklist for Human

• Toggle the theme button in the navbar — verify it switches between light/dark smoothly and the icon changes (moon → sun)
• Refresh the page after toggling — verify the theme preference persists via localStorage
• Navigate to article detail, profile, settings, editor, and auth pages in dark mode — verify all elements are styled correctly with readable contrast
• Test on a fresh browser (no localStorage) with OS-level dark mode preference — verify the app auto-detects and applies dark theme

Notes

• The existing unit tests fail on the base branch due to a missing zone.js dependency — this is a pre-existing issue unrelated to these changes
• The submodule file realworld/assets/theme/styles.css was intentionally not modified; all dark mode overrides live in src/dark-theme.css
• Build passes cleanly with no warnings

Link to Devin session: https://app.devin.ai/sessions/02d8be4cba814e5cb489872261c3ad55
Requested by: @WesternConcrete

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.