feat: sticky table of contents with scrollspy for canvas dashboards

[rohithreddykota]

Adds a persistent, minimap-style table of contents to canvas dashboards (view surfaces only; not the iframe embed).

• Derives entries from the headings (h1–h3) rendered inside markdown components of the active tab — no new section primitive; stays in sync as content/tabs change via a debounced MutationObserver.
• Scrollspy via a single IntersectionObserver (no scroll-event math); exactly one active section, with the highlight pinned during click-to-scroll so it doesn't flicker through intermediate sections.
• Collapsed rail of tick bars by default (dots on narrow viewports so they don't overlap content); hover/focus reveals the full list as an overlay flyout that never reflows the dashboard.
• Click smooth-scrolls (respecting prefers-reduced-motion) and updates the URL hash; deep-linking on mount is supported. Accessible: <nav aria-label>, real links, aria-current="location", focus rings, full heading as the accessible name.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

---

Developed in collaboration with Claude Code