fix(alerts/ui): page header, scroll, title preview, bell badge polish

Visual regressions surfaced during browser smoke:

1. Page headers — `SectionHeader` renders as 12px uppercase gray (a
   section divider, not a page title). Replace with proper h2 title
   + inline subtitle (`N firing · N total` etc.) and right-aligned
   actions, styled from `alerts-page.module.css`.

2. Undefined `--space-*` tokens — the project (and `@cameleer/design-system`)
   has never shipped `--space-sm|md|lg|xl`, even though many modules
   (SensitiveKeysPage, alerts CSS, …) reference them. The fallback
   to `initial` silently collapsed gaps/paddings to 0. Define the
   scale in `ui/src/index.css` so every consumer picks it up.

3. List scrolling — DataTable was using default pagination, but with
   no flex sizing the whole page scrolled. Add `fillHeight` and raise
   `pageSize`/list `limit` to 200 so the table gets sticky header +
   internal scroll + pinned pagination footer (Gmail-style). True
   cursor-based infinite scroll needs a backend change (filed as
   follow-up — `/alerts` only accepts `limit` today).

4. Title column clipping — `.titlePreview` used `white-space: nowrap`
   + fixed `max-width`, truncating message mid-UUID. Switch to a
   2-line `-webkit-line-clamp` so full context is visible.

5. Notification bell badge invisible — `NotificationBell.module.css`
   referenced undefined tokens (`--fg`, `--hover-bg`, `--bg`,
   `--muted`). Map to real DS tokens (`--text-primary`, `--bg-hover`,
   `#fff`, `--text-muted`). The admin user currently sees no badge
   because the backend `/alerts/unread-count` returns 0 (read
   receipts) — that's data, not UI.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/src/index.css

@@ -64,6 +64,14 @@
   --text-muted: #766A5E;
   /* White text on colored badge backgrounds (not in DS yet) */
   --text-inverse: #fff;
+
+  /* Spacing scale — DS doesn't ship these, but many app modules reference them.
+     Keep local here until the DS grows a real spacing system. */
+  --space-xs: 4px;
+  --space-sm: 8px;
+  --space-md: 16px;
+  --space-lg: 24px;
+  --space-xl: 32px;
 }
 
 [data-theme="dark"] {