Sonner: replace the JS runtime with a pure Rust/Leptos toast system

[krishpranav]

Description:

The Sonner docs were still hanging off a bundled JS runtime and lazy loader.
This patch moves the toast state machine, rendering, timers, stacking, and interaction flow into Rust/Leptos and cleans up the demo wiring around it.

What changed:

• Replaced the Sonner JS runtime with a Rust/Leptos implementation in app_crates/registry/src/ui/sonner.rs
• Removed Sonner asset loading from app/src/shell.rs
• Deleted public/components/sonner.js, sonner.min.js, sonner.css, and lazy_load_sonner.js
• Added reactive toast context, Rust click handlers, auto-dismiss timers, swipe dismissal, stacking, and variant handling
• Updated the Sonner demos so the default, positions, and variants previews render against the Rust implementation directly
• Added stable demo preview selectors and tightened the Sonner Playwright page objects around those demos
• Refreshed the Sonner registry markdown snapshots

Validation:

cargo build
cargo clippy -- -D warnings

cd e2e && PORT=3000 npx playwright test tests/components/sonner.spec.ts --workers=1 --grep "Loading variant should NOT have duration track"
cd e2e && PORT=3000 npx playwright test tests/components/sonner.spec.ts --workers=1 --grep "BOTTOM position: stacked toasts should expand UPWARD on hover|all toasts should have scale\\(1\\) when expanded|progress bar should resume after mouse leave"
cd e2e && PORT=3000 npx playwright test tests/components/sonner.spec.ts --workers=1 --grep "Sonner Positions Page"

The long full-file Playwright run against tests/components/sonner.spec.ts is still unstable in this local runner after extended sequential execution, but the Rust implementation, demo wiring, and the targeted Sonner slices above pass cleanly.

Fixes #29