feat(chat): custom DragonX emoji via the :drgx: shortcode

Add a DragonX custom emoji to chat. The emoji picker gains a DragonX tile (first
cell) that inserts the text ":drgx:"; message bodies containing it render the mark
inline via layoutChatBodyRich, which flows text words + the emoji image with word
wrap (plain bodies keep the tighter layoutChatBody path, so normal messages are
unaffected). Other clients simply show the literal ":drgx:" text — a portable
encoding with graceful degradation.

The emoji is the DragonX SVG rasterized once at fixed brand colors (crimson body,
white detail) via LoadTextureFromSvg — theme-independent so it looks identical for
sender and receiver — exposed as App::getDrgxEmojiTexture(). Emoji insertion is
factored into one insertToken() helper (space-prepend + on-chain byte cap), shared
by the DragonX tile and the Unicode emojis. Drag-to-select text is skipped on
":drgx:" messages (their inline layout doesn't match the plain-text hit-test
geometry); right-click "copy" still copies the whole message.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app.cpp

@@ -1361,6 +1361,13 @@ void App::handleGlobalShortcuts()
 // theme accent, so it re-rasterizes whenever the accent or the dark↔light variant changes.
 void App::ensureLogoTexture()
 {
+    // DragonX custom chat emoji (":drgx:") — the mark in FIXED brand colors (crimson body + white detail),
+    // rasterized once. Kept theme-independent so it looks identical for the sender and the receiver.
+    if (!drgx_emoji_tex_) {
+        util::LoadTextureFromSvg(embedded::kLogoDragonXSvg, 96, IM_COL32(0xD8, 0x26, 0x52, 0xFF),
+                                 IM_COL32(255, 255, 255, 255), &drgx_emoji_tex_, &drgx_emoji_w_, &drgx_emoji_h_);
+    }
+
     const bool wantDark = ui::material::IsDarkTheme();
     const ImU32 logoAccent = ui::material::Primary();
     if (logo_loaded_ && wantDark == logo_is_dark_variant_ && logoAccent == logo_accent_)