feat: origin-based namespaces + 10s bucket grid view

[Copilot]

Namespaces were UUID-only and the log viewer had no way to see all log levels side-by-side. This PR ties namespace identity to the connecting origin's hostname and adds a time-bucketed multi-column view.

Origin-based namespaces

Server (server/index.js)

• Logger connections now derive their namespace from the Origin header hostname (e.g. https://www.domain1.com → namespace www.domain1.com). Falls back to UUID when Origin is absent or unparseable (Node.js clients).
• activeLoggers: Set → activeLoggerCounts: Map<namespace, number> to support concurrent loggers from the same origin sharing one namespace.
• UUID_PATH_RE → NAMESPACE_PATH_RE — accepts both UUID slugs and domain-name slugs, with strict start/end alphanumeric validation to reject degenerate hostnames.

Frontend (App.tsx)

• getNamespaceFromPath() updated to use the same permissive pattern, enabling viewer routes like /www.domain1.com.

4-column bucket view

New BucketViewer.tsx

• Groups logs into 10-second windows and renders a sticky-header table: Time Bucket | ERROR | DEBUG | INFO | WARN. Empty cells show n/a. Search filter from FilterBar applies. Auto-scrolls to the latest bucket.

Header.tsx / App.tsx

• Added a ⊞ Bucket / ☰ List toggle in the header that switches between the new BucketViewer and the existing LogViewer.

10:00:00–10  │ err1, err2  │ debug1       │ n/a   │ n/a
10:00:10–20  │ err3        │ debug2       │ n/a   │ warn1