feat: add contentsByLocale to update inputPlaceholder per locale

[pepalonsocosta]

Description

Add a contentsByLocale prop to the webchat that allows bots to provide locale-specific
content overrides. The inputPlaceholder is the first supported field — it updates
automatically whenever the user's system_locale changes.

Context

The input placeholder was set once at module load time inside the static theme object, so unlike other locale-driven contents it never reacted to system_locale changes.

Approach taken / Explain the design

• Add WebchatLocaleContents type and contentsByLocale: Record<string, WebchatLocaleContents> wired through WebchatArgs, WebchatProps, WebchatApp, and DevApp down to the Webchat component
• updateSessionWithUser resolves contentsByLocale[system_locale].inputPlaceholder and calls updateTheme whenever system_locale is set, falling back to WEBCHAT_DEFAULT_CONTENTS.INPUT_PLACEHOLDER
• Extract the default placeholder string into WEBCHAT_DEFAULT_CONTENTS to avoid duplication

To document / Usage example

new WebchatApp({
  contentsByLocale: {
    en: { inputPlaceholder: 'Write a reply...' },
    es: { inputPlaceholder: 'Escribe tu pregunta...' },
    pt: { inputPlaceholder: 'Digite sua pergunta...' },
  },
})

Testing

The pull request...

• has unit tests
• has integration tests
• doesn't need tests because... [provide a description]

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[Iru89]

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