Allow Jellyfin HTTP for local and VPN servers

[lostf1sh]

Closes #30

This makes the Jellyfin login flow friendlier for setups where the server is only reachable over a trusted local network or VPN, like Tailscale.

What changed:

• If a Jellyfin server URL is entered without a scheme, local/VPN-looking hosts now default to http:// instead of always assuming https://.
• Kept https:// as the default for normal public hostnames.
• Added a small “Use HTTP / VPN” button on the Jellyfin login screen so users can explicitly switch/prefix the URL with HTTP.
• Updated the Jellyfin helper text to mention local/Tailscale HTTP support.
• Added unit test coverage for the URL normalization behavior.