fix(nix): correct flake src path and add dev shell (#9894)

The flake set `src = ./sources;` referencing a non-existent subdirectory,
so `nix build` and `nix develop` both failed evaluation. Point `src` at
the repo root and refresh `vendorHash` accordingly.

Add `devShells.default` with the Go toolchain, protobuf generators,
Node.js/bun for the React UI (`make react-ui`), and the linters used by
`make lint` (golangci-lint, gofumpt, goimports, staticcheck).

Assisted-by: Claude:claude-opus-4-7

Signed-off-by: Richard Palethorpe <io@richiejp.com>