fix(web): improve reply draft dialog accessibility

[thomasorgeval]

Summary

• Adds a visible label, helper text, placeholder, and stronger focus ring to the Reply Draft textarea.
• Adds an explicit Cancel action so users do not need to rely on the close icon or backdrop.
• Localizes the new dialog guidance in English and French.

Expected impact

• Improves screen-reader and keyboard clarity when composing or editing Reply Drafts.
• Makes the dialog feel more guided and forgiving without changing the publishing flow.

Verification

• pnpm biome format --write apps/web/src/app/pages/reply-inbox/components/reply-draft-dialog.component.html apps/web/src/app/pages/reply-inbox/components/reply-draft-dialog.component.ts apps/web/public/i18n/en.json apps/web/public/i18n/fr.json
• pnpm nx typecheck web --tui=false
• pnpm nx lint web --tui=false
• pnpm nx build web --configuration=production --tui=false