Add contextual error screen that preserves dashboard layout

[stylessh]

Summary

• Error screens now render inside the dashboard card content area instead of replacing the entire shell, preserving the topbar, card chrome, and bottombar.
• Errors show friendly, contextual messages based on type: 403 opens a "Configure access" prompt via the existing GitHub access dialog, 404 shows "Not found", plus handling for rate limits, network failures, and timeouts.
• GitHub's raw error detail is shown in a muted code box (with the HTTP method/URL suffix stripped).
• Renames remaining QuickHub references to DiffKit (user agent, review dismiss message).

Test plan

• Trigger a 403 error (e.g. access a repo without app installation) — should show "Access not configured" with "Configure access" button that opens the GitHub access dialog
• Trigger a 404 error — should show "Not found" with search icon
• Verify dashboard layout (topbar, card, bottombar) remains intact during any error
• Verify "Try again" button resets the error and re-fetches

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	diffkit	a7c64ed	Commit Preview URL Branch Preview URL	Apr 12 2026, 08:07 PM