chore: bump Go toolchain to 1.26.3 (#110)

Pin Go version to 1.26.3 across go.mod, CI workflows, and Dockerfiles
for canonical alignment with the rest of the luxfi/* stack.

docker/Dockerfile.local

@@ -1,5 +1,5 @@
 # Build stage
-FROM golang:1.26-alpine AS builder
+FROM golang:1.26.3-alpine AS builder
 
 RUN apk add --no-cache git make gcc musl-dev linux-headers bash
 

docker/Dockerfile.multichain

@@ -1,5 +1,5 @@
 # Multi-chain node builder
-FROM golang:1.26-alpine AS builder
+FROM golang:1.26.3-alpine AS builder
 
 ARG CHAIN_TYPE=platform
 

docker/Dockerfile.simple

@@ -1,4 +1,4 @@
-FROM golang:1.26-alpine AS builder
+FROM golang:1.26.3-alpine AS builder
 
 RUN apk add --no-cache git make gcc musl-dev linux-headers bash