feat(context-window): show current/max context usage per tab/model

Add a 'Context Window' sidebar view showing the live context occupancy
(latest request's input+output) against the model's maximum context
window, resolved dynamically from the models.dev catalog.

- core: models.dev catalog module (resolveContextLimit) with disk cache,
  TTL, stale-fallback + offline penalty memo; null for unknown models.
- api: GET /models/context-limit?provider=&modelId=.
- frontend: ContextWindowPanel + computeContextUsage helper; App resolves
  + caches the active model's max (anthropic/opencode-anthropic only);
  percent shown to 2 decimals; degrades to bare token count when max
  unknown.
- tests: core catalog (13), api route (3), frontend helper (6).

1 files changed, 85 insertions, 0 deletions

diff --git a/packages/frontend/src/lib/components/ContextWindowPanel.svelte b/packages/frontend/src/lib/components/ContextWindowPanel.svelte
new file mode 100644
index 0000000..6c7de05
--- /dev/null
+++ b/packages/frontend/src/lib/components/ContextWindowPanel.svelte
@@ -0,0 +1,85 @@
+<script lang="ts">
+import { computeContextUsage } from "../context-window.js";
+import type { CacheStats } from "../types.js";
+
+const {
+	cacheStats = null,
+	contextLimit = null,
+	tabTitle = null,
+	modelId = null,
+}: {
+	cacheStats?: CacheStats | null;
+	contextLimit?: number | null;
+	tabTitle?: string | null;
+	modelId?: string | null;
+} = $props();
+
+const usage = $derived(computeContextUsage(cacheStats, contextLimit));
+
+// As the window fills, escalate color: calm → warning → danger.
+function fillClass(pct: number): string {
+	if (pct >= 90) return "progress-error";
+	if (pct >= 70) return "progress-warning";
+	return "progress-success";
+}
+
+function fmt(n: number): string {
+	return n.toLocaleString();
+}
+
+const hasUsage = $derived((cacheStats?.last ?? null) !== null);
+</script>
+
+<div class="flex flex-col gap-3 flex-1 min-h-0 overflow-y-auto">
+	{#if !hasUsage}
+		<p class="text-xs text-base-content/50">
+			No context data yet. Send a message — the current context size appears
+			here after the first response.
+		</p>
+	{:else}
+		<div class="bg-base-200 rounded-lg p-2">
+			<div class="flex items-center gap-1.5 mb-2">
+				<span class="text-xs font-semibold">Context Window</span>
+				{#if tabTitle}
+					<span class="badge badge-xs badge-ghost">{tabTitle}</span>
+				{/if}
+				{#if usage.percent !== null}
+					<span class="badge badge-xs ml-auto">{usage.percent.toFixed(2)}%</span>
+				{/if}
+			</div>
+
+			<!-- Headline: current / max (or just current when max is unknown) -->
+			<div class="flex items-baseline gap-1.5">
+				<span class="text-lg font-mono font-semibold">{fmt(usage.current)}</span>
+				{#if usage.max !== null}
+					<span class="text-xs text-base-content/50 font-mono">/ {fmt(usage.max)}</span>
+				{/if}
+				<span class="text-xs text-base-content/40 ml-1">tokens</span>
+			</div>
+
+			{#if usage.percent !== null}
+				<progress
+					class="progress w-full h-2 mt-1.5 {fillClass(usage.percent)}"
+					value={usage.percent}
+					max="100"
+				></progress>
+			{:else}
+				<p class="text-xs text-base-content/40 mt-1.5">
+					Max context size unknown for this model.
+				</p>
+			{/if}
+
+			{#if modelId}
+				<div class="text-xs text-base-content/40 mt-1.5 truncate" title={modelId}>
+					{modelId}
+				</div>
+			{/if}
+		</div>
+
+		<p class="text-xs text-base-content/40">
+			Current context = the most recent request's prompt + output (what the
+			model actually held in its window that turn). Grows as the conversation
+			gets longer. Resets on reload.
+		</p>
+	{/if}
+</div>