fix(web): preview image deliverables (#149)

## Summary
- Render generated image outputs directly in the deliverables card and
an accessible full-size viewer.
- Keep preview fetches lazy, signed, bounded to 20 MiB, and isolated
from sandbox startup.
- Open generated images directly in the Files editor instead of
selecting their parent directory.

## Architecture
The web app exchanges the existing short-lived download capability for
an authenticated image blob. Object URLs stay browser-local and are
revoked on cleanup. File-open requests are transient UI state sent to
the trusted Code Server frame and acknowledged by the injected preview
bridge.

## Decisions
| Decision | Choice | Reasoning |
|---|---|---|
| Preview transport | Existing signed output URL | Preserves the R2
boundary and does not expose credentials |
| Loading | Intersection-observer lazy loading | Avoids fetching
off-screen images or waking the sandbox |
| Files handoff | Exact relative workspace path with frame
acknowledgement | Opens the file users selected and rejects traversal
paths |
| Viewer | Existing modal primitive | Preserves focus, Escape handling,
and screen-reader semantics |

## Edge cases
- Images over 20 MiB remain downloadable but are not loaded inline.
- Empty or non-image responses show a recoverable preview error.
- Unsafe filenames cannot be sent to the sandbox editor.
- Requests are scoped to the active thread and exact trusted iframe
origin.

## Checks
- [x] `pnpm turbo typecheck`
- [x] `pnpm turbo lint`
- [x] `pnpm turbo build`
- [ ] Production image deliverable QA after deployment

Author: iamjr15

apps/web/README.md

@@ -11,7 +11,12 @@ partial or corrupt transcript is never rendered as a duplicate assistant message
 
 Deliverable parts contain durable output identity and presentation metadata, never an expiring
 URL. A download click calls the authenticated gateway mint endpoint, validates its bounded response,
-and follows the resulting short-lived capability directly to the streaming response.
+and follows the resulting short-lived capability directly to the streaming response. Image
+deliverables lazily exchange that same capability for a bounded in-memory blob when they approach
+the viewport, render an inline thumbnail, and reuse the blob in an accessible full-size viewer.
+No image capability or object URL is persisted. Opening an image in Files switches to the project
+workspace and asks the trusted code-server bridge to reveal the exact generated asset; an already
+visible Files panel selects a newly generated image without waking a closed sandbox.
 
 Composer uploads always land in a writable project. If none is selected, choosing the first
 valid file creates and selects a general project named from that file. Files upload sequentially