feat: What's-new changelog dialog on first launch after an update (EN/FR)

Embeds changelog.json (per-version EN + FR notes, curated from the release's
commits). GetWhatsNew compares the stored last-seen version with the running
build and returns the notes for every newer version, once, then advances the
marker. A dialog shows them in the UI language on the first launch after update.
Seeded with the 0.20.5 notes.

changelog.go

@@ -0,0 +1,67 @@
+package main
+
+import (
+	_ "embed"
+	"encoding/json"
+	"strings"
+
+	"hamlog/internal/applog"
+)
+
+//go:embed changelog.json
+var changelogJSON []byte
+
+// ChangelogEntry is one release's user-facing notes, in English and French. It's
+// shown once on the first launch after an update (the "What's new" dialog),
+// derived from the release's commit history.
+type ChangelogEntry struct {
+	Version string   `json:"version"`
+	Date    string   `json:"date"`
+	EN      []string `json:"en"`
+	FR      []string `json:"fr"`
+}
+
+const keyChangelogSeen = "changelog.last_seen_version"
+
+func loadChangelog() []ChangelogEntry {
+	var out []ChangelogEntry
+	if err := json.Unmarshal(changelogJSON, &out); err != nil {
+		applog.Printf("changelog: parse failed: %v", err)
+	}
+	return out
+}
+
+// GetWhatsNew returns the changelog entries the operator hasn't seen yet — the
+// notes for every version newer than the last one they ran, up to the current
+// build — and records the current version as seen so it pops exactly once per
+// update. Empty when there's nothing new.
+func (a *App) GetWhatsNew() []ChangelogEntry {
+	if a.settings == nil {
+		return nil
+	}
+	lastSeen, _ := a.settings.GetGlobal(a.ctx, keyChangelogSeen)
+	cur := appVersion
+	a.setSettingGlobal(keyChangelogSeen, cur) // advance the marker now
+
+	out := []ChangelogEntry{}
+	for _, e := range loadChangelog() {
+		if strings.TrimSpace(e.Version) == "" {
+			continue
+		}
+		if versionLess(cur, e.Version) {
+			continue // don't preview notes for a version newer than we run
+		}
+		if lastSeen == "" {
+			// First run of the feature (fresh install, or upgrade from a build that
+			// predates it): show only the CURRENT version's notes, once.
+			if e.Version == cur {
+				out = append(out, e)
+			}
+			continue
+		}
+		if versionLess(lastSeen, e.Version) {
+			out = append(out, e)
+		}
+	}
+	return out
+}