Multi-site fleet page — consolidate features/rackManagement into features/fleetManagement

[flesher]

Follow-up to #368 (Fleet page scaffold).

Context

The Sites/Buildings work consolidated into features/fleetManagement/ (alongside MinerList, FleetLayout, SiteList, BuildingList, etc.). The rack surface still lives in features/rackManagement/ and predates the redesign — it remains an "old" sibling folder for what is now an integrated piece of the unified Fleet page.

Scope

Move everything under client/src/protoFleet/features/rackManagement/ into client/src/protoFleet/features/fleetManagement/:

• pages/RacksPage.tsx → fleetManagement/pages/RacksPage.tsx
• pages/RackOverviewPage.tsx (+ test) → fleetManagement/pages/
• components/RackCard/ → fleetManagement/components/RackCard/
• components/AssignMinersModal/ (full directory incl. RackPane, MinersPane, SearchMinersModal, ManageMinersModal, tests, stories) → fleetManagement/components/AssignMinersModal/
• components/RackHealthModule/ → fleetManagement/components/
• components/RackSlotGrid/ → fleetManagement/components/
• components/RackDetailGrid/ → fleetManagement/components/
• components/RackSettingsModal.tsx (+ stories) → fleetManagement/components/
• utils/{buildingFilterUrl,rackCardMapper,slotNumbering}.ts (+ tests) → fleetManagement/utils/

Then:

• Update every @/protoFleet/features/rackManagement/... import. Roughly 15 files import from this path today (grep -rln "@/protoFleet/features/rackManagement").
• Update routePrefetch.ts factory paths (importRacksPage, importRackOverviewPage).
• Remove the now-empty features/rackManagement/ directory.

Why defer this from #368

The move is mechanical but wide: ~30 files relocate plus every importer touches an import line. Bundling it with the Sites/Buildings refactor in PR #368 would have inflated the diff and made the substantive review (List-component adoption, query-string preservation, redirect race fix, etc.) harder to read. Splitting it off keeps each PR focused.

Out of scope

• Renaming any of the rack components themselves.
• Splitting AssignMinersModal (it stays as one directory; the redesign's "Add miners" reassignment flow on the Buildings tab will land elsewhere per Multi-site fleet page — per-row ellipsis menus on Sites + Buildings tabs #369).