fix(app): guard comment accessor in message timeline

[Hona]

Summary

• Guard comment() accessor with <Show> in the message timeline comment bubbles to prevent TypeError: Cannot read properties of undefined (reading 'path').

Root cause

The <Index each={comments()}> accessor can briefly return undefined during SolidJS reactive batch updates when the underlying sync.data.part[messageID] store changes. The createMemo(() => commentAccessor()) captures this undefined, and comment().path throws.

Stack trace

TypeError: Cannot read properties of undefined (reading 'path')
    at Object.fn (main-*.js)
    at runComputation (styles-*.js)
    at updateComputation
    at runTop
    at completeUpdates
    at runUpdates

[Copilot]

Pull request overview

Prevents a runtime crash in the session message timeline by guarding access to a comment object that can temporarily be undefined during SolidJS reactive updates.

Changes:

• Wrap each comment bubble’s rendering in <Show when={comment()}> to avoid reading properties from an undefined comment.
• Update comment bubble internals to reference the <Show>-provided accessor (c()) rather than the unguarded memo value.

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