feat(master): Ledger UI for Services and Agents (#18) (#29)

## Summary

- Rebuilds the master's **Services** and **Agents** pages per the "Ledger" design: sidebar shell with live nav counts, machine filter and Live indicator; dark/light theme (follows the OS, toggle overrides); embedded Schibsted Grotesk and JetBrains Mono (fontsource 5.3.0, OFL).
- Services: Swarm grouped per cluster with its machines under it, offline banner ("Agent offline since HH:MM", browser-local), Simple Icons derived from the image ref (letter circle fallback), existing badges and mode switch restyled, client-side search, machine filter and Icon / Icon + text / Text toggle.
- Agents: Online / Offline / Never connected / Revoked, master version in the header with outdated agent versions marked, revoke via popover instead of `confirm()`, "Add agent" as a three-step panel that completes when the agent says hello (SSE `agents`).
- Store: migration `0005_agent_cluster.sql` persists each agent's `swarm_cluster_id` (needed to list a cluster's managers and its offline state after a master restart).
- SSE keepalive is now a named `ping` event so the Live indicator also shows the connection is alive. `revokeAgent` now publishes `agents`.

Closes #18

## Notes

- Workers never report a cluster id (Docker only sets `Swarm.Cluster` on managers), so their Swarm role shows "—". Fixing that needs a new hello field.
- Master binary grows by about 2.16 MB, almost all Simple Icons 16.35.0 path data (gzipped JSON, regenerated with `go generate ./internal/master/web/icons`). The agent binary does not import it.
- Jobs, Job and Service detail pages only get the new shell; their own artboards are out of scope, as are Update buttons, Update all, header stats and the drift badge.
- Unchanged: CSRF on every POST, `Cache-Control: no-store` on the token response, 401 + `HX-Redirect` without a session, no `template.HTML` on agent- or user-controlled values, no Node build step.

## Test plan

- [x] `go test ./...`, `go test -race ./internal/master/...`
- [x] `go test -tags integration ./internal/master/store/` against Postgres 17
- [x] `docker build`
- [x] Browser check on fake data: both themes, 1440 px and 390 px, add-agent flow through connect
- [ ] Services grouping and icons against real stacks and a real Swarm cluster
- [ ] "Agent offline since" after stopping an agent
- [ ] Theme toggle, search and machine filter across reloads and mode changes
- [ ] Add agent end to end with a real agent, Copy buttons
- [ ] Phone at 390 px

https://claude.ai/code/session_01GAZsp3P7Qto6JnhipxWUJQ
Reviewed-on: #29

internal/master/web/icons/icons.go

@@ -0,0 +1,109 @@
+// Package icons maps container image references to brand icons from the
+// Simple Icons set (CC0, see LICENSE-simple-icons.md). Only the master's UI
+// imports it, so the agent binary does not carry the data.
+package icons
+
+import (
+	"bytes"
+	"compress/gzip"
+	_ "embed"
+	"encoding/json"
+	"strings"
+	"sync"
+
+	"github.com/shcizo/package-updater/internal/imageref"
+)
+
+//go:generate go run ./gen
+
+//go:embed simple-icons.json.gz
+var data []byte
+
+var (
+	once  sync.Once
+	paths map[string]string
+)
+
+// aliases maps a normalised image name to the Simple Icons slug that differs
+// from it.
+var aliases = map[string]string{
+	"postgres":    "postgresql",
+	"goauthentik": "authentik",
+	"traefik":     "traefikproxy",
+	"node":        "nodedotjs",
+	"mongo":       "mongodb",
+}
+
+// skipNamespaces are registry namespaces that name a publisher, not a product.
+var skipNamespaces = map[string]bool{"library": true, "linuxserver": true, "bitnami": true, "hotio": true}
+
+// suffixes are stripped from the last path segment to find the product name
+// ("authentik-server" is authentik).
+var suffixes = []string{"-server", "-web", "-api", "-worker", "-agent"}
+
+func load() {
+	once.Do(func() {
+		paths = map[string]string{}
+		zr, err := gzip.NewReader(bytes.NewReader(data))
+		if err != nil {
+			return
+		}
+		_ = json.NewDecoder(zr).Decode(&paths)
+	})
+}
+
+// Path returns the SVG path data (24x24 viewBox) of the icon with this Simple
+// Icons slug.
+func Path(slug string) (string, bool) {
+	load()
+	p, ok := paths[slug]
+	return p, ok
+}
+
+// Count returns how many icons are embedded.
+func Count() int {
+	load()
+	return len(paths)
+}
+
+// ForImage returns the slug of the icon that best matches the image
+// reference, or "" when there is none. It tries the last path segment, that
+// segment without a role suffix, and the namespace, in that order.
+func ForImage(ref string) string {
+	segs := strings.Split(imageref.Parse(ref).Repo, "/")
+	if len(segs) > 1 && imageref.IsRegistryHost(segs[0]) {
+		segs = segs[1:]
+	}
+	last := segs[len(segs)-1]
+	candidates := []string{last}
+	for _, s := range suffixes {
+		if base, ok := strings.CutSuffix(last, s); ok && base != "" {
+			candidates = append(candidates, base)
+			break
+		}
+	}
+	if len(segs) > 1 && !skipNamespaces[segs[len(segs)-2]] {
+		candidates = append(candidates, segs[len(segs)-2])
+	}
+	for _, c := range candidates {
+		slug := normalise(c)
+		if a, ok := aliases[slug]; ok {
+			slug = a
+		}
+		if _, ok := Path(slug); ok {
+			return slug
+		}
+	}
+	return ""
+}
+
+// normalise lowercases s and keeps only a-z and 0-9, the form of Simple Icons slugs.
+func normalise(s string) string {
+	var b strings.Builder
+	for _, r := range strings.ToLower(s) {
+		if r >= 'a' && r <= 'z' || r >= '0' && r <= '9' {
+			b.WriteRune(r)
+		}
+	}
+	return b.String()
+}