Canvas generation history defaults to the first take instead of the latest successful version #25

Canvas generation cards should default to the latest successful image/video take. The shared resolver currently trusts input order, while buildGenerationTakes supplies oldest-first order. This highlights Take 1 even when the main preview shows a newer take. In addition, browser generation completion preserves an older pinnedOutputId, so a previously selected take can keep the main preview on an old result after generating a new version.

Reproduction:

1. Generate two successful image or video outputs for a Canvas generation card.
2. With no explicit pin, inspect the main preview and selected take thumbnail: the thumbnail falls back to Take 1.
3. Select Take 1 and generate another version through the browser: completion retains the old pin.

Expected: the default preview, take highlight, version picker and resolved media reference agree on the newest usable successful output; explicit manual history selection remains possible. A newly completed browser generation should become the selected version, matching MCP generation completion. Pending/failed outputs must not replace usable successful media.

Proposed fix: make fallback resolution independent of caller ordering, reuse it in Canvas presentation, update the browser completion pin, and add regression coverage for image/video, out-of-order inputs, reloads, manual selection and pending/failed outputs. Timeline clips remain pinned to their concrete assets.