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, 16 insertions, 0 deletions

diff --git a/packages/api/src/routes/models.ts b/packages/api/src/routes/models.ts
index 03c079a..6a0f5dc 100644
--- a/packages/api/src/routes/models.ts
+++ b/packages/api/src/routes/models.ts
@@ -17,6 +17,7 @@ import {
 	listStoredCredentials,
 	refreshAccountCredentialsAsync,
 	resolveApiKey,
+	resolveContextLimit,
 	setApiKey,
 	validateAccountCredentials,
 } from "@dispatch/core";
@@ -161,6 +162,21 @@ modelsRoutes.get("/available", async (c) => {
 	return c.json({ models });
 });
 
+// Resolve a model's MAXIMUM context window (in tokens) from the models.dev
+// catalog. Returns `{ contextLimit: number | null }`; `null` means the model's
+// limit is unknown (unsupported provider, unknown model, or catalog offline),
+// which the frontend renders without a denominator/percentage.
+modelsRoutes.get("/context-limit", async (c) => {
+	const provider = c.req.query("provider");
+	const modelId = c.req.query("modelId");
+	if (!provider || !modelId) {
+		return c.json({ error: "provider and modelId query parameters are required" }, 400);
+	}
+
+	const contextLimit = await resolveContextLimit(provider, modelId);
+	return c.json({ contextLimit });
+});
+
 // List available Claude accounts with validated credentials
 modelsRoutes.get("/claude-accounts", async (c) => {
 	const candidates = resolveClaudeAccounts();