Split internal/web into subpackages before domain-admin growth.

Lay out view, auth, validate, and handlers under internal/web while keeping
the cmd/panel API unchanged; update roadmap and changelog for web-split closure.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/web/view/templates/system_log_body.html

@@ -0,0 +1,7 @@
+{{define "system_log_body"}}
+<div id="system-log-body" data-poll hx-get="/system-log/body" hx-trigger="load" hx-swap="outerHTML">
+  {{if .Error}}<p class="error">{{.Error}}</p>{{end}}
+  <span class="code">{{if .Lines}}{{range .Lines}}{{.}}
+{{end}}{{else}}No log lines yet.{{end}}</span>
+</div>
+{{end}}