P1-24: live dashboard — fleet summary tiles + host table

Server-rendered HTML view backed by:
  - new store.FleetSummary aggregating host counts + repo bytes +
    snapshot total + open alerts + last-24h job rollup in two queries.
  - GET /api/hosts (JSON list of hosts in the dashboard projection).
  - GET /api/fleet/summary (JSON aggregate, same shape as above).

The HTML page (web/templates/pages/dashboard.html) renders the four
summary tiles + host table directly from store data — no separate
fetch. Per-row state colour comes from .host-row.{degraded,failed,
offline} which paint a 3px left edge so problem hosts are scannable
without reading. HTMX is loaded into the base layout so per-row
"Run now" buttons can hx-post to /hosts/{id}/run-backup, a thin
HTML wrapper that funnels into a new dispatchJob helper shared
with the JSON /api/hosts/{id}/jobs endpoint.

Empty state (zero hosts) collapses to the "no hosts yet" prompt
with the + Add host CTA — matches the v1 mockup.

Template helpers (internal/server/ui/funcs.go) added for byte
formatting (412 GB / 3.7 TB), relative time (3m ago / 2d ago), and
comma grouping (1,847). Pure Go, no template-magic dependency.

Browser-verified end-to-end with seeded fixture data: five hosts
across all four states render with correct dots, accents, last-
backup pills, sizes, snapshot counts, alerts, tags, and the right
action button (Run now / Retry / Run first / View → / offline).

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

web/styles/input.css

@@ -161,6 +161,7 @@
   .host-row {
     display: grid; align-items: center;
     grid-template-columns: 24px 1.4fr 0.95fr 1.5fr 0.75fr 0.7fr 0.7fr 1.1fr 92px;
+    column-gap: 18px;
     padding: 11px 16px; font-size: 13px;
     border-left: 3px solid transparent;
   }