fix(app): text-shimmer undefined length

[Eric-Guo]

Issue for this PR

Closes #16473

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Fix below 500 error in desktop, 100% by Codex.app GPT 5.4

TypeError: Cannot read properties of undefined (reading 'length')
    at Object.fn (http://localhost:3000/@fs/Users/guochunzhong/git/oss/opencode/packages/ui/src/components/text-shimmer.tsx:41:37)
    at runComputation (http://localhost:3000/node_modules/.vite/deps/chunk-VL2SL2GY.js?v=a409caaf:734:22)
    at updateComputation (http://localhost:3000/node_modules/.vite/deps/chunk-VL2SL2GY.js?v=a409caaf:717:3)
    at Object.readSignal (http://localhost:3000/node_modules/.vite/deps/chunk-VL2SL2GY.js?v=a409caaf:650:67)
    at get style (http://localhost:3000/@fs/Users/guochunzhong/git/oss/opencode/packages/ui/src/components/text-shimmer.tsx:64:39)
    at assign (http://localhost:3000/node_modules/.vite/deps/chunk-5C2DUIQJ.js?v=a409caaf:759:24)
    at Object.fn (http://localhost:3000/node_modules/.vite/deps/chunk-5C2DUIQJ.js?v=a409caaf:725:28)
    at runComputation (http://localhost:3000/node_modules/.vite/deps/chunk-VL2SL2GY.js?v=a409caaf:734:22)
    at updateComputation (http://localhost:3000/node_modules/.vite/deps/chunk-VL2SL2GY.js?v=a409caaf:717:3)
    at createRenderEffect (http://localhost:3000/node_modules/.vite/deps/chunk-VL2SL2GY.js?v=a409caaf:236:8)

How did you verify your code works?

It works in my machine.(TM)

Screenshots / recordings

No UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

If you do not follow this template your PR will be automatically rejected.

[Copilot]

Pull request overview

This PR fixes a runtime TypeError ("Cannot read properties of undefined (reading 'length')") in the TextShimmer component. The error occurs when props.text is undefined at runtime despite being typed as string, which can happen in SolidJS when the reactive value feeding the prop hasn't resolved yet.

Changes:

• Wraps props.text in a createMemo with a ?? "" fallback to guard against undefined values
• Replaces all direct props.text accesses with the safe memoized text() accessor
• Extracts the shared len computation into its own memo, eliminating duplication between shimmerSize and shimmerDuration

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[cyberprophet]

This is the more complete fix of the two PRs addressing this bug (the other being #16476).

What's good:

• Wrapping in createMemo(() => props.text ?? "") is the right SolidJS pattern — the fallback is memoized and reactive, so it only re-evaluates when props.text changes
• Extracting len into its own memo eliminates the duplicated computation between shimmerSize and shimmerDuration — good cleanup
• Replacing all props.text references (including aria-label and render slots) with text() is thorough

One minor note: The len memo name is a single word per the project style guide — that's correct. The text memo name is also fine.

Root cause context for reviewers: props.text is typed as string but arrives as undefined at runtime during initial reactive evaluation (e.g. before i18n resolves). This is the same class of bug as #16459 / #16467 in session-header.tsx where current() can be undefined when options() is empty. A fix for that is incoming as a separate PR.

Prefer this PR over #16476 — it's more complete.