Make Site / Building / Rack columns sortable in the miner list (server-side sort support)

## Context

The miner selection modal (Search / Manage miners) now shows Model, IP, **Site, Building, Rack**, and Groups columns (#673). Model, IP, and Name are sortable; **Site, Building, and Rack are not**, because the server has no sort support for placement columns.

## Why it's blocked today

The miner list is server-paginated (50/page), so sorting must be server-side — a client-only sort would only reorder the current page. But the server's sort whitelist doesn't include placement:

- `server/internal/domain/stores/sqlstores/device_query_fragments.go` — `sortExpressions` supports only Name, IP, MAC, Model, Hashrate, Temperature, Power, Efficiency, Firmware, WorkerName.
- `SORT_FIELD_LOCATION` exists in `common.v1.SortField` but is explicitly *reserved/unimplemented* (`renaming_test.go`).
- The fleet miner-list page (`MinerList/sortConfig.ts`) doesn't offer these either — so this would be net-new capability, not just wiring.

## Scope

Add server-side sort for **Site, Building, Rack** (Group sort is lower value and multi-valued — call it out but likely skip):

- **Proto:** add `SORT_FIELD_SITE`, `SORT_FIELD_BUILDING`, `SORT_FIELD_RACK` to `common.v1.SortField` (or repurpose `SORT_FIELD_LOCATION` as a composite site→building→rack ordering). Regen Go/TS/SDK.
- **Server:** add `sortExpressions` entries with the JOINs to the placement/collection + site/building tables, plus matching keyset-pagination expressions (`device_sort.go`) so cursor paging stays correct. Handle NULLs (unplaced miners) with `NULLS LAST`.
- **Client:** extend `SORT_FIELD_BY_COLUMN` in `MinerSelectionList.tsx` (and optionally `MinerList/sortConfig.ts` for the fleet page) so the site/building/rack column headers become sortable.
- **Tests:** server sort + keyset-pagination tests; client sort wiring.

## Notes

- Decide composite vs. discrete: a single `LOCATION` sort (site → building → rack → slot) may match operator intent better than three independent sorts. Worth a quick UX call before implementing.
- Group is multi-valued per miner (a miner can be in several groups), so "sort by group" is ambiguous — recommend leaving Group unsortable.

## Related

- Follows #673 (miner selection modal: columns + nested filters + assignable-only toggle). Client already lists Site/Building/Rack columns; this makes them sortable.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Start with common.v1.SortField and its renaming_test.go coverage, then read server/internal/domain/stores/sqlstores/device_query_fragments.go, device_sort.go, and the client sort mapping in MinerSelectionList.tsx. Compare existing server sort and keyset-pagination tests before resolving the composite-versus-discrete location choice. Done means regenerated clients, correct NULLS LAST cursor sorting, sortable client columns, and passing server and client tests.

Written by the indexing model from the issue text.