feat: add in-app release notes and what's new dialog

PROJECT_KNOWLEDGE.md

@@ -18,6 +18,7 @@ Backend:
 - `/` renders the active calculator template.
 - `/help` renders the authenticated Help landing page.
 - `/help/user-manual` renders `docs/de/user_manual.md` server-side inside the RollCalc Help layout.
+- `/help/recent-changes` renders `docs/de/recent_changes.md` server-side inside the RollCalc Help layout.
 - `/help/user-manual/screenshots/<path:filename>` serves German manual screenshots from `docs/de/screenshots/`.
 - `/static/<path:filename>` is intended to serve static files behind Basic Auth.
 - `/api/health` returns health/version information.
@@ -77,6 +78,30 @@ Rendering rules:
 
 The Help pages use the same HTTP Basic Auth protection as the calculator.
 
+### Recent Changes and What's New
+
+Full user-facing release notes are maintained in:
+
+```text
+docs/de/recent_changes.md
+```
+
+The calculator header links directly to `/help/user-manual`. The Help landing page lists only existing documents, currently User Manual and Recent Changes.
+
+The startup What's New dialog uses `build_info.version` and is suppressed when the version is empty or `unknown`. The last acknowledged version is stored only in browser `localStorage` under:
+
+```text
+rollcalc_last_seen_version
+```
+
+The short dialog item list is maintained separately in:
+
+```text
+static/recent_changes.json
+```
+
+This JSON is a compact summary for the dialog, not the complete release history. Keep it aligned with the most relevant current entries from `docs/de/recent_changes.md`.
+
 ## Authentication
 
 RollCalc uses HTTP Basic Auth. User entries in `BETA_USERS` have this shape: