fix(renderer): make code entrance animation type line-by-line

[tongshu2023]

Summary

Fixes #531 — the code element's entrance animation typed all lines simultaneously instead of line-by-line.

Root cause

Exactly the ordering issue the report hypothesized:

1. First render: animStates is an empty Map (it was only populated via a setTimeout(..., 0) inside an effect).
2. Every CodeLineRow therefore mounts with animState === undefined → isNewLine === false → useState(!isNewLine || typingDelay === 0) latches mounted = true. The typingDelays map is also empty at this point, so every row gets typingDelay = 0.
3. One tick later the states arrive, isNewLine flips to true for every row — but mounted is already latched true, so the delayed-mount path (setTimeout(() => setMounted(true), typingDelay)) is a no-op, and the mounted && isNewLine effect fires setTyping(true) for all rows in the same tick.

Fix

Compute the first-render 'typing' states synchronously in the useState initializer, so rows see their animState and cumulative typingDelay on the very first render: line 0 types immediately, each later row renders null until its stagger delay elapses. The effect now only handles subsequent line edits (inserted/replaced), with prevLinesRef seeded to the initial lines and the first effect run skipped.

Behavior note: previously, toggling animate false→true marked every line inserted (because prevLinesRef started empty). With the ref seeded, a toggle no longer replays the entrance — flagging in case that old behavior was intentional.

Scope note: packages/@maic/renderer/src/elements/code/BaseCodeElement.tsx has the identical first-render pattern, but #706 is currently rewriting that exact effect (prev-line map). To avoid conflicting with it, this PR only fixes the app copy; happy to port the same fix to the package copy once #706 lands.

Verification

• Code-path trace above; tsc --noEmit, eslint, prettier --check all clean.
• No component-level test added: vitest.config.ts only includes tests/**/*.test.ts in a node environment (no jsdom/RTL infrastructure for .tsx behavior tests). If you'd like one, I can propose a minimal jsdom setup in a separate PR.

🤖 Generated with Claude Code

[wyuc]

LGTM. Verified the code block now reveals line-by-line on first paint instead of all at once. Good call scoping this to the app copy to avoid colliding with #706's package rewrite. Thanks!