Settings flags a player that skips SSL certificate checks, and other fixes

Fixes

• Notifications in the web interface show their success, error or info icon, and the Load Average card on System Info shows its icon when the load is steady. Both spaces were empty.
• In the Edit asset dialog for a web page, the "Auto-refresh interval (seconds)" and "Custom HTTP headers" labels no longer sit on top of their example text.
• In Settings > Import content, each platform's import button sits beside its description instead of under it.
• The "Try again" and "Copy ffmpeg command" buttons in the asset dialog have a button style. They showed without one.

Improvements

• When "Verify SSL certificates" is off, Settings shows a "Not secure" badge next to it, so a player that skips certificate checks is easy to spot. It appears as soon as you switch the setting off, before you save.
• The disk chart on System Info no longer shows used space in red. It uses the same colors as the memory chart, and a storage problem shows up in the new storage card instead.
• The import pages under Settings > Import content use the web interface's own colors. Their progress bar was a blue used nowhere else.
• On a board with little memory, the "Low-RAM mode" note on System Info's Memory card is in a darker amber that's easier to read on white.

For contributors

• The web interface's colors, fonts and sizes are defined once, in src/anthias_server/app/static/src/tailwind.css, with dark values in css/theme-dark.css next to it. The older SCSS copy, which had been overriding them, is gone, and tests fail if one comes back.
• A design-system page at /_design/ shows every color and component, in light and dark. It exists only in development builds and tests (DEBUG or IS_TEST), so production images don't have the route.
• bin/shoot_pages.py screenshots the web interface pages in both themes and at several window sizes, without the Docker stack.
• The host agent lists network addresses through the kernel's rtnetlink interface with the Python standard library. The netifaces and netifaces2 packages are gone, and so is python3-netifaces in the viewer image. uv sync --group test works on a plain development machine, where netifaces used to fail to build.
• The release and hardware-testing guides for coding agents in .claude/skills add a burn-in run on the test boards before a release is published.
• Tests cover the power and storage checks, SMART parsing, the network address listing and the design-system page.