fix(transcript): scroll to latest message on first open

[mpmisha]

What

Opening the Transcript panel (⌘+⌥+T / title-bar button) on a long conversation used to dump you at the very first message instead of jumping to the most recent one.

Why

The initial fetch in TranscriptPanel.tsx called scrollToBottom() inside a single requestAnimationFrame immediately after setMsgs(next). React hadn't actually rendered the new bubbles yet at that point, so scrollHeight ≈ clientHeight and scrollTop = scrollHeight effectively landed at the top. After the real render committed, the view stayed parked at message #1.

How

• Added a useLayoutEffect keyed on [open, aiSessionId, msgs] that performs the first-render jump. It runs after layout (so scrollHeight is correct) but before paint (no visible flicker / jump).
• A new initialScrolledForRef tracks which session id has already had its initial scroll-to-bottom done, so the jump fires exactly once per session/open cycle and live polls don't re-trigger it.
• The existing live-poll "glue to bottom" behavior is preserved for follow-up updates when the user is already at the bottom; only the racy initial || short-circuit in the fetch path was removed.

Testing

Manual:

• Opened the transcript on a long (200+ message) session — now lands on the most recent message.
• Re-opened on the same session multiple times — consistent.
• Switched panes / followed focus to a different session — jumps to that session's latest message.
• Scrolled up, waited for a live update — view stays put (no auto-scroll). Scrolled back to bottom, new message arrived — glues to bottom. Both unchanged from before.
• Opened the search bar and parked on a hit — live updates don't yank the view away. Unchanged.

npx tsc --noEmit shows no new errors in the touched file (a handful of pre-existing errors in unrelated files are untouched).

assistance: agentic-cli
type: bug
agent-tool: copilot-cli
agent-model: claude-opus-4.7
work-item: n/a