Add Canva as a one-click OAuth MCP connector preset

[TheGreatAxios]

Summary

• Adds Canva to MCP_PRESETS (CL-6647): https://mcp.canva.com/mcp, OAuth 2.0 with CIMD + DCR — no vendor pre-registration needed, so the existing /plugins "Connect apps" one-click flow (already wired for Notion, Linear, Sentry, etc.) picks it up automatically.
• Canva has no simple-icons brand mark yet, so its card falls back to the existing initial-letter tile — same convention already used for Granola and Sumble.
• Updates the hardcoded preset-list assertion in workflows/assistant/test/manager-tools-scenario.test.ts to include Canva.

Part of CL-6639's plugin batch 1. Stacked before #cl-6647-calendly (Calendly, next in the batch).

Test plan

• WORKBENCH_CHECK_SINCE=origin/main bun run typecheck
• WORKBENCH_CHECK_SINCE=origin/main bun run test
• WORKBENCH_CHECK_SINCE=origin/main bun run lint
• Browser: confirm the Canva tile renders on /plugins with a Connect affordance and reaches Canva's OAuth consent redirect