Apply per-client bootloader set overrides

[hcaldicott]

Summary

• apply bootloader-set precedence as client, client group, then global
• pass the requesting PXE MAC into the proxyDHCP bootfile selector
• carry client-specific selections into TFTP/HTTP with set-qualified paths
• retain existing unqualified global behaviour and built-in file fallback
• document the built-in proxyDHCP requirement for MAC-specific selection

Root cause

The client and client-group bootloader_set fields are persisted and exposed
by the UI, but the boot-serving path only consults GetActiveBootloaderSet().
Consequently, the per-client override advertised in the v0.1.13 release notes
has no effect during PXE boot.

proxyDHCP knows the requesting MAC, while the later TFTP request only carries a
filename. This change resolves the effective set during proxyDHCP and advertises
a path such as bootloader-sets/custom/bootimus.efi, preserving that choice for
the subsequent transfer.

Unqualified requests continue to resolve through the global active set, so
external DHCP configurations retain their existing behaviour.

Validation

• gofmt -l .
• go vet ./...
• go test ./...
• go test -race ./internal/proxydhcp ./internal/server
• go build ./...
• GOOS=windows GOARCH=amd64 go build ./...
• GOOS=darwin GOARCH=arm64 go build ./...

Fixes #116. Related global bootloader/proxyDHCP wiring: #78 and #96.

[hcaldicott]

GPT 5.6 was used in this PR, however I have completed a manual (human) review of the codebase. I (personally) am happy with the changes here and feel they're as minimal as possible to implement the given functionality. I am still pulling apart the unit tests though and validating them.