e2e: Yuki dock persist + SSE EventSource consumer (Playwright)

[h0lybyte]

Tracker: #11583

Problem

Yuki shipped Phase A through D.2 with zero browser tests. The dock relies on every fragile primitive at once: `transition:persist`, ClientRouter swap, dynamic `import('./yuki-dock')`, `localStorage` state for collapsed/expanded + float mode + history + greeted flag, an `EventSource` consumer for SSE chat, and a Three.js + VRM render loop. Regressions in any of these would not be caught.

Acceptance

`apps/kbve/astro-kbve/e2e/yuki.spec.ts` covers:

• Dock FAB renders on initial paint, no JS in the critical path
• Click FAB → dock expands, panel mounts (`[data-kbve-yuki-panel-mounted="true"]`)
• Navigate to another route via ClientRouter → dock survives (persist) with state intact
• Submit a prompt → `/api/v1/yuki/chat?q=…` SSE stream is consumed, bubble fills, `event: done` terminator wired
• First-open greet flag (`kbve:yuki-dock:greeted`) flips to `'1'` exactly once across page loads on the same device
• Float toggle → layer detaches and persists across one nav swap

3D-mode tests are out of scope here — Three.js + WebGL in headless mode is its own rabbit hole; track separately if needed.