feat: add exact snapshot preview

[luhe19001]

What this changes

This PR adds an exact, local preview of the session snapshot that a future Team sharing flow will upload. The preview is created before any data leaves the machine and from the selected saved session revision.

This PR introduces	This prevents
A pre-upload preview of the exact selected session snapshot	Sending session data to a server before the user can review it
One canonical serialization path whose unchanged bytes are displayed by the preview and can be passed to the future uploader	A separate preview payload builder drifting from the data that is actually uploaded
A controlled preview dialog with an explicit development-only test entry point	Showing Team sharing controls to users of the local-only application

What users see

The normal local-only experience is unchanged: no sharing button is shown.

When the development test entry point is enabled, a session has a Team sharing preview action. The dialog is labeled PREVIEW ONLY · TEAM FEATURE and shows:

• the canonical JSON snapshot and its byte size and content hash;
• repository identity, frozen cost, and any unpriced models;
• fields that were redacted or truncated, including known credential patterns; and
• data that is structurally excluded, including raw transcripts, assistant reasoning, tool output, file diffs, commands, environment variables, and unresolved local paths.

Other sensitive text can remain in the bounded firstPrompt, so the dialog shows its exact value for review rather than claiming that every possible credential format is removed.

The preview blocks approval when the snapshot is invalid, uses an unsupported format, no longer matches the selected revision, or exceeds the 256 KiB payload limit. This PR does not provide an approval or upload action.

Implementation boundary

The local collector owns this preview because both the session source and the existing session-snapshot/v1 serializer are local. The collector serializes the snapshot once, and the preview decodes those same bytes for display. A future Team sharing flow must upload those unchanged, already-reviewed bytes.

This PR includes:

• the local /api/share-preview endpoint for one session ID and revision;
• the versioned snapshot-preview/v1 response and its fail-closed states;
• the controlled React preview dialog and byte-parity checks; and
• privacy documentation and public/server fixture conformance.

This PR does not include Team eligibility, authentication, destination selection, multi-session selection, consent submission, upload, retry, or success navigation. Those belong to the production Team sharing flow that will consume this preview.

Try the preview

1. From collector/, run make run.
2. In another terminal, from frontend/, run npm run dev.
3. Open http://127.0.0.1:5173/?team-preview=1.
4. Select a session and click Team sharing preview.

Remove ?team-preview=1 to confirm that the local-only UI contains no sharing action.

Verification

• go test ./...
• go vet ./...
• 19 frontend tests
• frontend build, lint, and format checks
• snapshot fixture validation against coSlash Server commit c0fabc8

Follow-up requirement

The production Team sharing flow must open this controlled preview only after the user enters a sharing flow and selects what to share. It must upload the exact canonical bytes the user reviewed, rather than rebuilding the snapshot.