all: upgrade to Go 1.26rc2 and modernize codebase

This commit upgrades the codebase from Go 1.25.5 to Go 1.26rc2 and
adopts new language features.

Toolchain updates:
- go.mod: go 1.25.5 → go 1.26rc2
- flake.nix: buildGo125Module → buildGo126Module, go_1_25 → go_1_26
- flake.nix: build golangci-lint from source with Go 1.26
- Dockerfile.integration: golang:1.25-trixie → golang:1.26rc2-trixie
- Dockerfile.tailscale-HEAD: golang:1.25-alpine → golang:1.26rc2-alpine
- Dockerfile.derper: golang:alpine → golang:1.26rc2-alpine
- .goreleaser.yml: go mod tidy -compat=1.25 → -compat=1.26
- cmd/hi/run.go: fallback Go version 1.25 → 1.26rc2
- .pre-commit-config.yaml: simplify golangci-lint hook entry

Code modernization using Go 1.26 features:
- Replace tsaddr.SortPrefixes with slices.SortFunc + netip.Prefix.Compare
- Replace ptr.To(x) with new(x) syntax
- Replace errors.As with errors.AsType[T]

Lint rule updates:
- Add forbidigo rules to prevent regression to old patterns

.golangci.yaml

@@ -45,6 +45,16 @@ linters:
             Import "github.com/juanfont/headscale/hscontrol/util/zlog/zf" and use
             constants like zf.NodeID, zf.UserName, etc. Add new constants to
             hscontrol/util/zlog/zf/fields.go if needed.
+        # Forbid ptr.To - use Go 1.26 new(expr) instead
+        - pattern: 'ptr\.To\('
+          msg: >-
+            ptr.To is forbidden. Use Go 1.26's new(expr) syntax instead.
+            Example: ptr.To(value) → new(value)
+        # Forbid tsaddr.SortPrefixes - use slices.SortFunc with netip.Prefix.Compare
+        - pattern: 'tsaddr\.SortPrefixes'
+          msg: >-
+            tsaddr.SortPrefixes is forbidden. Use Go 1.26's netip.Prefix.Compare instead.
+            Example: slices.SortFunc(prefixes, netip.Prefix.Compare)
       analyze-types: true
     gocritic:
       disabled-checks: