feat(version): bake release tag into binary; About card and /health field

The version tag previously existed only in git and the Docker image tag —
the running app had no way to report what build it was. Version is now
injected at build time via -ldflags into internal/version.Version (defaults
to "dev" for local builds), passed by the release workflow as the
APP_VERSION Docker build arg from the pushed tag.

Surfaced in Admin → Settings → About (extensible card for future rows like
disk space) and in the /health JSON response, so deployments can be
verified with curl alone.

templates/admin_settings.templ

@@ -1,6 +1,9 @@
 package templates
 
-import "fmt"
+import (
+	"bookhoard/internal/version"
+	"fmt"
+)
 
 templ AdminSettings(user User, systemConfig map[string]string, scanSettings ScanSettingsData, liveGroups []SettingGroup, restartGroups []SettingGroup, errorMessage string) {
 	<!DOCTYPE html>
@@ -111,6 +114,7 @@ templ AdminSettings(user User, systemConfig map[string]string, scanSettings Scan
 				@ScanSettingsSection(scanSettings)
 				@TunableSettingsSection(liveGroups, false)
 				@TunableSettingsSection(restartGroups, true)
+				@AboutSection()
 			</div>
 		</main>
 	</body>
@@ -255,3 +259,18 @@ templ TunableSettingRow(e SettingEntry) {
 		<span id={ "status-" + e.Key } class="text-xs w-24 text-right" style="color: var(--text-secondary)"></span>
 	</div>
 }
+
+// AboutSection shows build metadata, rendered at the bottom of the System
+// Settings page. Add future rows (disk space, uptime, Go version, …) to the
+// list below.
+templ AboutSection() {
+	<div class="mt-6 card p-6">
+		<div class="flex items-center gap-2 mb-4">
+			@Icon("info", "h-5 w-5 shrink-0")
+			<h3 class="text-lg font-semibold" style="color: var(--text-primary)">About</h3>
+		</div>
+		<div class="space-y-2 text-sm" style="color: var(--text-secondary);">
+			<p><span class="font-medium uppercase tracking-wide text-xs" style="color: var(--text-secondary)">Version:</span> { version.Version }</p>
+		</div>
+	</div>
+}