Fix Chromecast Integration on Stash app

[dude-stash]

Description

Chromecast doesn't work: you press Cast, the device connects, but the TV only shows the Cast logo and nothing plays, with no error.

Two reasons:

1. Stash used the Video.js Chromecast tech. A tech is a player backend, so it sends the TV the same stream the browser is playing (Direct stream), and Stash reports that as video/mp4 even when the file is MKV. Chrome accepts that; a Chromecast cannot play MKV at all, so it just waits.
2. The URL is http://localhost:9999. The Chromecast downloads the video itself, and for the Chromecast, localhost means the Chromecast.

This is why the workarounds in the issue help some people and not others: casting from an IP fixes the address, DNS tricks fix the hostname, but neither fixes the MKV problem.

What this does:

Stash now talks to the Google Cast framework directly from the scene player, so it can choose what to send:

• Pick the stream from the real video and audio codecs, not from the MIME type. Direct stream only for H.264 + AAC in mp4, otherwise the original transcode, otherwise HLS.
• Replace localhost with a LAN address of this machine. The browser can't read network interfaces, so the server reports them in a new systemStatus.localIPs field.
• Keep the page on HTTPS or http://localhost, since the Cast SDK won't start anywhere else — only the media URL changes.
• If a scene can't be cast, show an error instead of hanging.

Signed URLs from #6529 still work, so authentication isn't affected.

UI-wise, nothing changes — same button, same place, same icon, same setting. @silvermine/videojs-chromecast is removed (Stash draws the button itself now), which also removes the only UI lockfile dependency that wasn't from npm — it was a GitHub tarball that fails behind a strict proxy, so pnpm install now works there too.

Known issue: Chrome draws its own Cast button in the video's top-left corner that casts the tab, not the video. That comes from the browser, not Stash — disableRemotePlayback didn't hide it, so I left it for a follow-up.

Next: TV-side player controls (play/pause/seek) and simultaneous browser+TV playback, then AirPlay (same localhost problem).

Related Issue

Fixes #4136.

Testing

• 29 new unit tests for stream selection and URL rewriting (added Vitest, since the UI had no JS test runner; it now runs as part of pnpm run validate, so CI picks it up with no workflow change)
• make test: 34 packages, no failures
• make it: pass
• golangci-lint v2.11.4 (same version as CI): 0 issues
• pnpm run validate: biome, stylelint, tsc, format, tests all pass
• UI build: pass

Manual testing with a real Chromecast device:

• MKV scene: plays on the TV (first seconds take a while due to the transcode)
• H.264/AAC mp4 scene: plays with no transcode
• Chromecast setting toggled off: no cast button shown, cast_sender.js not loaded
• Control bar looks the same as before

Screenshots

No visual changes — the Cast button keeps the same icon, placement, and behavior from the user's perspective.

Checklist

• I have read and understood the Contributing document.
• I have read and understood the AI Usage Policy document.
• I have made corresponding changes to the documentation (if applicable).

AI Usage Disclosure

• I have used AI tools to assist with this pull request, and I have disclosed the tools and how I used them below.

I used Claude Code to help implement this feature and write the documentation in this PR. I reviewed, tested, and take responsibility for all of the code and the manual Chromecast testing described above.

Additional Context

None.