feat: iOS blue-bubble chat feed anchored to the bottom

[joeblau]

Summary

Reworks the main-screen chat feed to read like a chat app.

• Bottom scroll anchor — .defaultScrollAnchor(.bottom) on the ScrollView, so the newest message sits at the bottom of the screen on first paint and stays pinned as chat streams in.
• iOS blue bubbles — each incoming message renders through a new MessageBubble: a secondary author + platform caption above a content-hugging systemBlue bubble (18pt continuous corners, white body text), left-aligned with a trailing Spacer(minLength: 40) so short messages stay short and long ones wrap without truncating.
• Smarter follow — the old unconditional scroll-to-last is replaced with an isAtBottom-gated animated follow (tracked via .onScrollGeometryChange), so a viewer scrolled up reading history is no longer yanked to the bottom on every new message.
• Accessibility — each row folds into a single VoiceOver element ("<author> on <platform>: <text>").
• Empty-state ContentUnavailable overlay and per-row .id(message.id) preserved.

How it was built

A 5-agent workflow: 3 independent designs (faithful-iMessage / modern-clean / robust-a11y) → judged & synthesized (winner grafted the accessibility+follow engine onto the iMessage caption) → adversarial SwiftUI review → SHIP.

Known tradeoffs (reviewed & accepted)

• White-on-blue body text is ~4.0:1 contrast, just under WCAG AA (4.5:1) — this is Apple's exact iMessage pairing and matches the "blue bubble" request. Easy to nudge darker/bolder if strict AA is wanted.
• The gated animated follow is occasionally a visual no-op (the anchor already instant-follows); harmless, no fight between the two mechanisms.

Testing

• xcodebuild build -scheme Stream -destination 'generic/platform=iOS' → BUILD SUCCEEDED.
• Changes are app-target UI only; StreamCore test suite unaffected.
• Not yet visually verified on device (device-only iOS 26/27 build).

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