Help: Fn+h lists the keys of the screen you're on, and no screen names its keys any more (#69)

Fn+h on any screen, text fields included, and ? outside Text Entry, open a
panel over the content area: the screen's own keys, then the ones that work
everywhere. Every App declares its keys for the state it is in (pages,
viewers, dialogs and text fields answer for themselves); the App manager
opens the panel and takes every key while it is open.

About 30 hint lines are gone, from every App. What stays on a screen is
state. The first-start Setup keeps its hints and teaches the key; a device
set up before gets one Toast, once. The guide and the FAQ open with it.
`key help` over the consoles.

476 host tests (8 new). Checked on the device with key help and screenshots:
the Launcher, all nine Apps and several of their states. 8.5 KB of flash and
40 bytes of static RAM. Decisions Q196 to Q203 in docs/milestones/U1.md.

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

src/apps/gnss_app.cpp

@@ -80,11 +80,11 @@ void GnssApp::draw(Canvas& c) {
         return;
     }
     sky_ ? drawSky(c) : drawPosition(c);
-    c.setFont(&fonts::small);
-    c.setTextColor(theme::kMuted);
-    c.setTextDatum(top_right);
-    c.drawString(sky_ ? "Tab: position" : "Tab: sky", area.w - 3, area.y + area.h - 9);
-    c.setTextDatum(top_left);
+}
+
+void GnssApp::help(std::vector<KeyHelp>& out) const {
+    out.push_back({"Tab", sky_ ? "the position" : "the sky"});
+    out.push_back({"r", gnss_.tracking() ? "stop the Track" : "record a Track"});
 }
 
 void GnssApp::drawPosition(Canvas& c) {
@@ -139,15 +139,12 @@ void GnssApp::drawPosition(Canvas& c) {
     c.setFont(&fonts::small);
     if (gnss_.tracking()) {
         c.setTextColor(theme::kWarning);
-        std::snprintf(line, sizeof line, "REC %d point%s, %s  r: stop", gnss_.trackPoints(),
+        std::snprintf(line, sizeof line, "REC %d point%s, %s", gnss_.trackPoints(),
                       gnss_.trackPoints() == 1 ? "" : "s", duration(now - gnss_.trackStartMs()).c_str());
     } else if (!refusal_.empty() && now - refusalMs_ < 4000) {
         c.setTextColor(theme::kWarning);
         std::snprintf(line, sizeof line, "%s", refusal_.c_str());
-    } else {
-        c.setTextColor(theme::kMuted);
-        std::snprintf(line, sizeof line, "r: record a Track");
-    }
+    } else line[0] = 0;
     c.drawString(line, 4, area.y + area.h - 9);
 }