fix(json-chat): restore Chat-mode auto-scroll on streaming + send

[frenchie4111]

Summary

Regression introduced in #84 (1634f1d): Chat mode (json-mode) stopped auto-scrolling to the bottom — neither on streaming token deltas nor when the user sent a message.

PR #84 added the entriesHydrated gate to suppress the empty-state flash, but on a fresh chat mount (entries=[], !busy, entriesHydrated=false) the entire ternary returns null. That leaves the scroll container with no firstElementChild, so the useLayoutEffect that attaches the ResizeObserver — mounted once with empty deps — finds nothing to observe and never re-runs after entries hydrate. From that point on, streaming token deltas grow the content but never snap-to-bottom.

Fix: wrap the conditional in a stable min-h-full flex flex-col div so the RO always has a firstElementChild to attach to. The empty-state's previous min-h-full referenced the scroll container; with the wrapper in between, switch it to flex-1 so it fills the wrapper's column and the sparkle card stays centered.

Test plan

• npx electron-vite build passes
• npx vitest run passes (3 stale-out/ failures are unrelated to this change — source tests green)
• Manual: brand-new Chat tab — sparkle empty-state still centers correctly
• Manual: send a message — chat snaps to bottom as tokens stream in
• Manual: existing chat with history — lands at the bottom on mount
• Manual: scroll up mid-stream — position stays put (user-scroll-up gesture still respected)

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