fix(build-info): ship VERSION in backend image, add dialog blurb + fallback (#958)

[dolho]

Summary

Two problems behind the "every field shows unknown" Build Info dialog:

1. version always unknown on dev/prod. Backend reads version from /app/VERSION, but the file was only present via a docker-compose.yml bind mount (./VERSION:/app/VERSION:ro). Prod compose has no such mount and the Dockerfile never COPYed the file, so version defaulted to "unknown" everywhere except local-dev-with-bind-mount.
2. No operator guidance when fields are missing. Even after Show Trinity version in header + git commit/branch detail in Settings #926 added the build-args pipeline, an out-of-band Docker build or a missed start.sh run produced a wall of "unknown" rows with no explanation of what Build Info is or how to populate it.

Changes

• docker/backend/Dockerfile — COPY ../../VERSION /app/VERSION so the file ships in the image. Dev's bind mount still overrides for hot edits.
• src/frontend/src/composables/useBuildInfo.js — expose isMissing computed (true when every metadata field equals "unknown").
• src/frontend/src/components/NavBar.vue (Build Info modal) — short blurb under the title; gray-bg fallback note ("Build metadata not available — rebuild with scripts/deploy/start.sh to populate.") shown only when isMissing.
• src/frontend/src/views/Settings.vue (Build Info section) — same fallback note in the loading/error/data state chain. Existing blurb retained.

Related to #958

Test plan

Verified via Playwright (headless Chromium, route mock for missing state):

• Backend rebuilt without bind mount → /app/VERSION present in image (docker run --rm --entrypoint cat trinity-backend /app/VERSION → 0.9.0).
• GET /api/version returns "version": "0.9.0" instead of "unknown".
• NavBar modal renders blurb and populated fields. Screenshot.
• Settings → General → Build Info renders populated fields. Screenshot.
• Route-mocked GET /api/version with all fields = "unknown" → modal shows fallback note + blurb; Settings panel shows fallback note. Screenshots.
• isMissing correctly false when version populated but git fields unknown (intentional — partial provenance is still useful, not a "missing metadata" state).

🤖 Generated with Claude Code

[vybe]

LGTM — verified VERSION file exists at repo root (0.9.0), the COPY matches the established ../../ build-context pattern, and CI 'build' check passes (Dockerfile compiles). isMissing is correctly singleton-scoped and both consumers unwrap .value properly. Dev bind mount retained for hot edits. Validated via /validate-pr.