Add widget kit, input layer with Compose Key, Launcher and demo apps

Steps 5 and 6 of M0.
- lib/ui (host-tested): ListModel, LineEditor (UTF-8, byte-limited),
  wrapText, ToastQueue, DialogModel
- lib/input (host-tested): KeyMapper turns keyboard state into logical
  keys (Fn arrows, ` Back, Fn+` Home) and composes accents with opt
- src/ui: off-screen 16-bit frame pushed in one transfer (fixes the
  flicker), Status Bar, list, text, line editor, dialog, Toast widgets,
  Notifier (Toast + beep + LED), vendored public-domain Latin-1 fonts
- Apps: minimal Launcher, Widget demo, Diagnostics (SD erase now
  behind a confirmation dialog)

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

lib/ui/src/utf8.h

@@ -0,0 +1,53 @@
+#pragma once
+
+#include <cstdint>
+#include <string>
+
+namespace roro::utf8 {
+
+inline bool isContinuation(unsigned char c) { return (c & 0xC0) == 0x80; }
+
+inline std::string encode(uint32_t cp) {
+    std::string out;
+    if (cp < 0x80) {
+        out += static_cast<char>(cp);
+    } else if (cp < 0x800) {
+        out += static_cast<char>(0xC0 | (cp >> 6));
+        out += static_cast<char>(0x80 | (cp & 0x3F));
+    } else if (cp < 0x10000) {
+        out += static_cast<char>(0xE0 | (cp >> 12));
+        out += static_cast<char>(0x80 | ((cp >> 6) & 0x3F));
+        out += static_cast<char>(0x80 | (cp & 0x3F));
+    } else {
+        out += static_cast<char>(0xF0 | (cp >> 18));
+        out += static_cast<char>(0x80 | ((cp >> 12) & 0x3F));
+        out += static_cast<char>(0x80 | ((cp >> 6) & 0x3F));
+        out += static_cast<char>(0x80 | (cp & 0x3F));
+    }
+    return out;
+}
+
+// Byte index of the character start before / after `pos`.
+inline size_t previous(const std::string& s, size_t pos) {
+    if (pos == 0) return 0;
+    do pos--;
+    while (pos > 0 && isContinuation(static_cast<unsigned char>(s[pos])));
+    return pos;
+}
+
+inline size_t next(const std::string& s, size_t pos) {
+    if (pos >= s.size()) return s.size();
+    do pos++;
+    while (pos < s.size() && isContinuation(static_cast<unsigned char>(s[pos])));
+    return pos;
+}
+
+// Longest prefix of at most `maxBytes` that doesn't cut a character.
+inline size_t clampToBoundary(const std::string& s, size_t maxBytes) {
+    if (s.size() <= maxBytes) return s.size();
+    size_t pos = maxBytes;
+    while (pos > 0 && isContinuation(static_cast<unsigned char>(s[pos]))) pos--;
+    return pos;
+}
+
+}  // namespace roro::utf8