feat(metrics): consume contextSize — current context-usage readout

Backend context-size handoff: re-pin [email protected] / [email protected]
(+ re-mirror .dispatch reference snapshots). Thread the optional contextSize
through core/metrics (done fold + durable + selectCurrentContextSize: latest
turn's defined value, undefined=>unknown never 0, durable-wins-over-live).
Chat store exposes currentContextSize; ContextSizeBadge renders
"N tokens in context" / "context size unknown" above the composer.
GLOSSARY: add context size / context window. 533 tests green.

1 files changed, 20 insertions, 0 deletions

diff --git a/src/features/chat/ui/ContextSizeBadge.svelte b/src/features/chat/ui/ContextSizeBadge.svelte
new file mode 100644
index 0000000..475d54f
--- /dev/null
+++ b/src/features/chat/ui/ContextSizeBadge.svelte
@@ -0,0 +1,20 @@
+<script lang="ts">
+	import { formatContextSize } from "../../../core/metrics";
+
+	let {
+		contextSize,
+	}: {
+		// The conversation's current context size (tokens occupied), or `undefined`
+		// ("unknown") when no finalized turn has reported one yet. Never `0` for the
+		// unknown case — `formatContextSize` renders a placeholder instead.
+		contextSize: number | undefined;
+	} = $props();
+
+	const label = $derived(formatContextSize(contextSize));
+</script>
+
+<div class="px-4 pb-1 text-xs opacity-60" aria-live="polite">
+	<span title="The model's max context window is not reported yet — current usage only.">
+		{label}
+	</span>
+</div>