feat(web): merge Fleet into a server-filterable Dashboard (UI nav Phase 1)

[offendingcommit]

Summary

Implements Phase 1 of the UI navigation rework spec
(docs/superpowers/specs/2026-06-02-ui-navigation-rework.md, PR #55): merge the
single-instance Dashboard and the multi-instance Fleet view into one
server-aware Dashboard.

• The Dashboard now lists every workspace across every configured server,
  labelled <workspace> (<server>), with a server filter (default "All servers").
• Cross-server aggregate cards (workspaces, conclusions, healthy/unreachable)
  reuse computeFleetAggregates — the former Fleet metrics.
• Opening a workspace activates its server (if not active) then drills into the
  existing /workspaces/$workspaceId detail route — no new routes needed.
• Per-server data fan-out lives in a new ServerWorkspaceRows child component
  (one per server → rules-of-hooks safe; mirrors the proven FleetRow pattern).
• Fleet removed from the sidebar nav.

Files

• packages/web/src/components/dashboard/Dashboard.tsx — rewritten as the unified parent.
• packages/web/src/components/dashboard/ServerWorkspaceRows.tsx — new per-server child.
• packages/web/src/components/layout/Sidebar.tsx — drop Fleet from TOP_NAV.
• packages/web/src/test/dashboard.test.tsx — new (renders the route, asserts the
  <server> labels + the server filter narrowing).

Validation

• make ci-web green (lint + typecheck + tests + build).
• dashboard.test.tsx renders the real Dashboard through the router (mocked transport)
  and asserts: both servers' workspaces appear labelled (Neo)/(Iris); the filter
  lists each server; selecting one narrows to it.
• Not yet done: manual browser check against ≥2 live servers — recommend eyeballing
  via make dev-web (or make up). The drill-down activate-then-navigate path is logic-
  tested but not exercised in a real browser here.

Deferred (intentional)

• The /fleet route + FleetDashboard/FleetRow removal is left for a follow-up:
  removing it churns fleet.test.tsx, which PR feat(web): eliminate browser CORS via header-driven /api proxy #54 also edits — doing it here would
  create a needless merge conflict. /fleet stays reachable by URL (just unlinked)
  until feat(web): eliminate browser CORS via header-driven /api proxy #54 lands, then a small cleanup PR removes the route + dead components.
• Phases 2–3 (Server as a navigable route level + outline drill-down; Fleet as an
  ops/settings view) still need the brainstorming pass per the spec.

Independent of #54–#57 (touches none of their files).