Keys: one table file for the device's help panel and the website's key tables (#72)

Every screen's keys are constant tables in lib/core/src/app_keys.h (52 of
them, each under an `// id: Title` comment). An App's help() picks the table
of the state it is in. site/tools/gen_dev_docs.py reads the same file and
writes site/data/keys.toml; the `keys` shortcode shows a screen's tables on
its guide page, and /guide/keys/ shows all of them.

The Site job fails when the data file is out of date or a page asks for a
table that doesn't exist, and now also runs when app_keys.h changes. A key
added to an App shows up on the website without anyone editing a page.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

src/apps/demo_app.cpp

@@ -1,3 +1,4 @@
+#include "app_keys.h"
 #include "demo_app.h"
 
 #include "ui/widgets.h"
@@ -26,12 +27,9 @@ void DemoApp::notify(const char* text, NotificationLevel level) {
 }
 
 void DemoApp::help(std::vector<KeyHelp>& out) const {
-    if (dialog_) return help::dialog(out);
-    switch (page_) {
-        case Page::Menu: help::list(out, "try the widget"); break;
-        case Page::Text: out.push_back({"; .", "scroll"}); break;
-        case Page::Editor: help::textEntry(out, "show the text as a Toast"); break;
-    }
+    if (dialog_) return keys::add(out, keys::kDialog);
+    if (page_ == Page::Editor) return keys::add(out, keys::kText);
+    if (page_ == Page::Menu) keys::add(out, keys::kDemo);
 }
 
 bool DemoApp::onKey(const KeyEvent& e) {