The floating AI-chat header badge summed metadata.usage (AI SDK
totalUsage, all steps) across every assistant row, showing the
cumulative tokens SPENT — which grows each turn as history is re-sent.
Replace it with the conversation's CURRENT context size.
- server: persist metadata.contextTokens in streamText onFinish from the
final-step `usage` (inputTokens + outputTokens ≈ current context
window occupancy); keep usage: totalUsage for back-compat/fallback
- client: derive the badge from the most recent assistant row's
contextTokens (fallback to that row's usage total for older chats)
instead of summing all rows
- types: add metadata.contextTokens to IAiChatMessageRow
- i18n: rename badge label "Tokens used in this chat" -> "Current
context size" (en-US)
No DB migration needed (metadata is a JSON column).
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