refactor(session): extract live runtime context

[setkyar]

Summary

• Extracted the live session page shell and bootstrapping into focused helpers for model hydration, stored layout, annotations, and runtime startup so SessionPage.svelte stays mostly declarative.
• Added an explicit session runtime context for the live model, navigator, reconciliation hook, and content runtime while keeping documented window compatibility shims for remaining consumers.
• Updated chat, live reload, tree navigation, and palette helpers to prefer the runtime context, and made content runtime wiring disposable on unmount.
• Added Vitest coverage for the new helpers plus a source-level export boundary test to catch live-only imports earlier.

Related issue

N/A

Type of change

• feat — new feature
• fix — bug fix
• docs — documentation only
• refactor — code change that neither fixes a bug nor adds a feature
• style — formatting / UI styling, no behavior change
• test — adding or updating tests
• chore — build, tooling, or maintenance

Live vs. Export

• Not applicable — this PR doesn't touch session rendering
• Considered both the live app and the export snapshot
• Kept internal/ui/embedded/ in sync with web/src/session/ changes — not needed; this changes live Svelte orchestration and tests only
• No live-only chrome (Vite scripts, active composer, SSE/API) leaked into export

Testing

• make check passes (test + build + vet) — not run
• Frontend tests (vitest) cover the change: cd web && npm test -- --run
• Go tests (go test ./...) cover the change
• UI changes verified in a browser — not run