feat(ui): stick-to-bottom autoscroll on the run page

[BatLeDev]

Add stick-to-bottom autoscroll to the run page log view: while the user is at the
bottom of a running run, the view follows new log lines as they stream in; any
upward scroll (or wheel-up) stops following, and scrolling back to the bottom
resumes it.

What changed

• New ui/src/composables/use-follow-bottom.ts — useFollowBottom(logCount, isActive)
  pins the document to the bottom on each new log while following and the run is
  still streaming. Uses VueUse useScroll (1px bottom tolerance, no manual
  threshold) plus a wheel listener to catch wheel-up when the page itself can't
  scroll.
• runs/[runId].vue: replaces the one-way :model-value="[steps.length - 1]"
  expansion-panel binding with a v-model="openPanels" recomputed by a watch on
  the step count (latest panel only while following, otherwise the user's
  selection minus removed steps), so panels no longer slam shut on every streamed
  log and manual toggles persist.

Why: the run log view didn't follow the live tail, and the previous panel
binding recomputed on every log line, collapsing panels mid-read.

Regression risks:

• Panel open/close semantics changed. Previously the last panel was force-opened
  on every render; now state is recomputed only when the step count changes.
  While following, non-latest panels collapse on each new step; when not
  following, user toggles persist. Confirm this is the intended UX.
• The page carries data-iframe-height (auto-height d-frame embed). pinToBottom
  sets the inner window scroll position, which is a no-op when the parent
  frame scrolls — so tail-following may not visually work inside an auto-height
  embed (the wheel-up "stop following" still works). Confirm whether the run page
  is embedded in that mode and whether following is expected to work there.
• following initializes to true even for completed runs; benign because
  pinning is gated by isActive() and the initial panel state (last open)
  matches prior behavior.