User message navigator: dot-based scroll UI, remove up button

[gary149]

This PR replaces the old “ScrollToPreviousBtn” with a new dot‑based user message navigator that makes it easy to jump between user messages in long chats.

What’s in

• New src/lib/components/UserMessageNav.svelte:
  • Shows only when there are >2 user messages.
  • Uses IntersectionObserver to highlight the message nearest the top of the scroll container.
  • Tap a dot to smooth‑scroll to that user message with a small top offset so it’s not glued to the viewport.
  • Accessible: listbox/options, ARIA labels, focus ring.
  • Mobile: visible and positioned above the bottom “scroll to bottom” button.
  • Styling: semi‑opaque rail, inactive dots with opacity, active dot filled.
• ChatWindow.svelte integration: remove up button usage and mount UserMessageNav above the existing ScrollToBottomBtn.

Implementation notes

• Observers are non‑reactive locals to avoid reactive loops; updates coalesced by IntersectionObserver.
• MutationObserver watches only for message add/remove (not subtree text) to stay efficient while messages stream.

Out of scope

• Left unrelated svelte‑check errors untouched; this PR is UI‑focused and avoids server/migration code changes.

Happy to tweak sizes/spacing or thresholds if you prefer different ergonomics.