refactor(ui): centralize canvas state and AppMark semantics

[Harry19081]

Summary

• centralize session-scoped Canvas derivation and mutations in useCanvasForTurn and the session store helpers
• remove the useCanvasPreviewForSession compatibility shim and replace duplicated state reads/mutations across chat, streaming, pinned actions, session sync, and tab rendering
• strengthen Canvas lifecycle/hook coverage and include the architecture/UI audit notes
• refactor AppMark to use clearer inline semantics, memoized rendering, reusable path constants, and optional accessible title support

Why

Canvas preview state was being derived and mutated from multiple consumers, which made session boundaries and visibility rules easier to drift. This extraction keeps those rules in one place.

AppMark currently has no production caller, so that commit has no user-visible effect until the component is used.

Existing overlap

The Canvas portion overlaps with the broader work in #281. This PR is a smaller, independently validated extraction; reviewers can choose whether to land it separately or fold it into #281.

The updater and pill work are intentionally excluded because equivalent PRs already exist.

Validation

• pnpm typecheck
• targeted ESLint on changed TypeScript/TSX files
• Canvas lifecycle and hook suites: 53 tests passed
• git diff --check
• architecture and frontend UI audit reports included

No standalone AppMark behavioral test was added because the component currently has no caller.