Multi site: New-rack create flow can seed an oversized all-miner selection (render freeze before save-time cap)

[flesher]

Background

From the Codex security review on PR #551 (MEDIUM). The "New rack" create flow seeds the new rack with the operator's current miner selection. In all-selection mode this resolves the full fleet set.

Problem

handleCreateNewLaunch in client/src/protoFleet/features/fleetManagement/components/MinerActionsMenu/MinerReparentPicker.tsx (~:688-717) passes the full resolved ids straight into the rack seed:

if (kind === "rack") {
  createFlow.launchCreateRack({ minerIds: ids, conflictCount: ... });
}

resolveAllModeIds only caps at MAX_MINERS (~10k), not at rack capacity (12×12 = 144 max). And unlike the other two paths, launchCreateRack in FleetCreateFlowProvider.tsx (~:109-118) applies no cap — launchCreateBuilding/launchCreateSite both call batchCapError(seed) first (~:163), but the rack launch goes straight to openRackSettings(seed). Capacity is only validated later at Save, while MinersPane renders one row per seeded miner.

So a large all-mode selection can render thousands of rows and freeze/severely degrade the browser before the operator ever reaches the save-time capacity guard, even though a rack holds at most 144 slots.

Impact

• Browser freeze / degradation on large fleets when launching "New rack" from an all-mode selection.

Proposed fix

Cap the rack seed before mounting ManageRackModal (mirroring the building/site batchCapError gate):

• Reject selections larger than the chosen rack capacity once settings are picked, or at least larger than the absolute max rack capacity (144), and surface a clear toast.
• Apply the cap in launchCreateRack and/or handleCreateNewLaunch so MinersPane never renders an unbounded seed.

Notes

• Severity MEDIUM (frontend reliability); not exploitable.
• Refs: PR Multi-site UI hardening: optional rack zone, tab null states, create-new parent flows #551, Codex security review (finding: "New-Rack Launch Can Render an Oversized All-Miner Selection"). Location flagged: MinerReparentPicker.tsx:699.

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