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.h

@@ -479,6 +479,10 @@ public:
     // Coin logo texture accessor (DragonX currency icon for balance tab)
     ImTextureID getCoinLogoTexture() const { return coin_logo_tex_; }
 
+    // DragonX custom chat emoji (the ":drgx:" shortcode) — the mark in fixed brand colors, lazily
+    // rasterized. Used by the emoji picker tile + inline in chat bubbles.
+    ImTextureID getDrgxEmojiTexture() const { return drgx_emoji_tex_; }
+
     /**
      * @brief Reload theme images (background gradient + logo) from new paths
      * @param bgPath Path to background image override (empty = use default)
@@ -1085,6 +1089,8 @@ private:
     bool logo_loaded_ = false;
     bool logo_is_dark_variant_ = true; // tracks which variant is currently loaded
     ImU32 logo_accent_ = 0;            // theme accent the SVG logo was last rasterized with (re-render on change)
+    ImTextureID drgx_emoji_tex_ = 0;   // ":drgx:" custom chat emoji (fixed brand colors) — rasterized once
+    int drgx_emoji_w_ = 0, drgx_emoji_h_ = 0;
 
     // Coin logo texture (DragonX currency icon, separate from wallet branding)
     ImTextureID coin_logo_tex_ = 0;