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

lib/input/src/key_mapper.h

@@ -23,7 +23,7 @@ struct RawKeys {
 
 // Turns keyboard state changes into logical KeyEvents: only newly pressed keys produce events;
 // Fn + ; . , / are arrows, and so are ; . , / alone when no text is being entered; ` is Back and
-// Fn + ` is Home; the Compose Key (opt) followed by an accent and a letter types the accented
+// Fn + ` is Home; Fn + h is Help anywhere, and so is ? when no text is being entered; the Compose Key (opt) followed by an accent and a letter types the accented
 // letter (opt ' e -> é).
 class KeyMapper {
    public: