feat(docker): add Slint UI plugin to demo image

[staging-devin-ai-integration]

Summary

Adds the Slint UI plugin to Dockerfile.demo so it ships in the demo image alongside all other core plugins. This completes the release checklist for the Slint plugin (merged in #239).

Changes to Dockerfile.demo:

• Stage 13 (slint-builder): New multi-stage build that compiles libslint.so with libfontconfig1-dev for font discovery in the software renderer.
• Runtime deps: Adds libfontconfig1 and fonts-dejavu-core so Slint can discover and render fonts at runtime.
• Plugin copy: Copies libslint.so into /opt/streamkit/plugins/native/.
• Sample files: Copies samples/slint/ (watermark, scoreboard, lower_third .slint files) into the image so the existing sample pipelines (video_slint_watermark.yml, video_moq_slint_scoreboard.yml) work out of the box.
• Header comment: Updated to mention Slint alongside Pocket TTS and Supertonic.

Marketplace release: The Slint plugin is already registered in marketplace/official-plugins.json. After merging, trigger the Marketplace Release workflow (workflow_dispatch on main) from the GitHub Actions tab to build the bundle, create the plugin-slint-v0.1.0 GitHub release, and publish the registry update.

Review & Testing Checklist for Human

• Verify the slint-builder stage compiles successfully in a Docker build (or wait for CI if there's a Docker build job)
• Confirm libfontconfig1 + fonts-dejavu-core are sufficient runtime deps for Slint text rendering (no missing .so at load time)
• After merge, trigger Actions → Marketplace Release → Run workflow on main to publish the plugin-slint-v0.1.0 bundle

Notes

• The Slint plugin has no ML models, so no model download step is needed (unlike other plugin stages).
• The build stage follows the same pattern as pocket-tts-builder and supertonic-builder (pure Rust, no sherpa-onnx dependency).
• marketplace/official-plugins.json was verified current — running generate_official_plugins.py produces no diff.

Link to Devin session: https://staging.itsdev.in/sessions/af568ab70825478fa1f96dc27bfe1b74
Requested by: @streamer45

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[staging-devin-ai-integration]

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