feat(web): merge Fleet into a server-filterable Dashboard

Implements Phase 1 of the UI navigation rework (docs/superpowers/specs/
2026-06-02-ui-navigation-rework.md): the Dashboard now lists every workspace
across every configured server as <workspace> (<server>), filterable by server,
with cross-server aggregate cards (reusing computeFleetAggregates). Opening a
workspace activates its server then drills into the existing detail route.
Per-server fan-out lives in a ServerWorkspaceRows child (rules-of-hooks safe,
mirrors FleetRow). Fleet removed from the sidebar nav; the /fleet route is left
intact for now (full removal deferred to avoid churning fleet.test.tsx while #54
is open).

Author: offendingcommit