[109f] HealthBar instance switcher (Combobox wired to /api/instances)

[fedorovvvv]

Sub-issue of #109.

Goal

Replace the static <span class="project"> in HealthBar with a Combobox that lists running instances and switches between them by changing the URL.

Scope

• Replace template/src/widgets/health-bar/ui/HealthBar.svelte brand block:
  • If 1 instance → keep current / {project} static label (no clutter).
  • If ≥2 instances → render Combobox (mono variant, sm size) listing all instances.
• Items: \${projectName} (primary) + \${host}:\${port} (secondary, mono).
• Current instance: highlighted/disabled in list.
• On select: window.location.replace('http://' + selectedInstance.id) — instance ID = host:port.
• Data source: new /api/instances endpoint (added by 109d).
• Polling: piggyback on existing healthPoller cadence (10s).

Forgeplan

• PRD-029 + RFC-024

Acceptance

• 3 instances running → all 3 appear in Combobox; selecting any other navigates browser to that instance.
• 1 instance → no Combobox, current behavior preserved (visual nil change).
• Type-check 0, smoke green, rule 24 grep clean.

Depends on

• 109d (registry + /api/instances)
• 109e (Combobox primitive)

[fedorovvvv]

Merged via #118 (commit a6fcaef).