fix: allowlist opusContext1mEnabled in SettingsUpdateSchema (#78)

Same root cause as the thinkingEffort fix in #73: the schema is
.strict(), so unknown keys in PUT /api/settings are rejected with
INVALID_INPUT and the toggle never persists. Verified live:
pre-fix returned {"errorCode":"INVALID_INPUT"}, post-fix accepts.

The frontend has been reading and writing this key for a while
(settings-ui.js:336, :1137; session-ui.js:331), so saves were
silently failing — users never noticed because the load path
falls back to false on missing keys.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/web/schemas.ts

@@ -268,6 +268,7 @@ export const SettingsUpdateSchema = z
     tunnelEnabled: z.boolean().optional(),
     tabTwoRows: z.boolean().optional(),
     agentTeamsEnabled: z.boolean().optional(),
+    opusContext1mEnabled: z.boolean().optional(),
     thinkingEffort: z.string().max(20).optional(),
     // UI visibility
     showFontControls: z.boolean().optional(),