docker: stabilize web-builder paths and bump builder Go to 1.26
Two related Dockerfile correctness fixes uncovered while validating the
review pass:
1. The web-builder stage used WORKDIR /web with vite's relative
outDir ../internal/server/web/dist resolving to /internal/server/web/dist
at the container root — working only by relative-path coincidence.
Mirror the host repo layout inside /src so the path is /src/internal/server/web/dist
in both stages, making the COPY --from explicit and stable against any
future WORKDIR or outDir change.
2. golang:1.25-alpine no longer compiles the codebase: the auxilia/culpa
dep uses errors.AsType which is in 1.26+. Bump the builder image to
golang:1.26-alpine and the go.mod directive from 1.25.0 to 1.26.0
so they agree on the actual minimum.
Verified: docker build (full multi-stage) green; produced binary runs.