Retry initial project load and surface failures in the sidebar

[justinleveck]

Summary

• AppContext fetched projects/repositories exactly once on mount with no retry, and the resulting error state was never read by any component (confirmed by grepping every useApp() call site).
• On a docker compose restart, the client's first request can beat the server to accepting connections - depends_on only orders container startup, it doesn't wait for readiness. That single failed fetch then silently stranded the user on an empty "No projects yet.", which reads exactly like lost data even though the server's persisted db.json was untouched the whole time. Creating a new project was the only thing that happened to trigger a fresh fetch (via handleAddProject's refreshData() call), which is why adding one "unlocked" the old data.
• Retries the initial load up to 4 times with increasing backoff (1s, 2s, 3s) before giving up, and shows a Retry banner in the sidebar if it still fails, instead of a silent empty state.

Test plan

• tsc -b, eslint, and full vitest suite (115/115) pass on client/
• Manually verified in-browser: killed the server, reloaded - error banner with Retry appeared after retries exhausted; brought the server back up and confirmed a fresh page load auto-recovers within the retry window, and clicking Retry after the banner appears recovers immediately, restoring the previously "missing" project without needing to create a new one