feat(chat): render emoji in messages and user text (Q12)

Merge a monochrome Noto Emoji subset into the text fonts so chat messages, the
composer, contact names, and memos render emoji (😀 🎉 ❤ 🔥 👍 …) instead of
tofu.

- Enable IMGUI_USE_WCHAR32 (imconfig.h): emoji live above the BMP (U+1F300+), so
  16-bit ImWchar literally can't address them. This widens ImWchar build-wide;
  the only ImWchar uses in-tree are glyph-range arrays and one BMP private-use
  codepoint, so nothing else is affected. Tests + full build pass.
- Bundle res/fonts/NotoEmoji-Subset.ttf — the OFL monochrome Noto Emoji (color
  CBDT/COLR fonts can't be rasterized by ImGui's stb_truetype) pinned to wght=400
  and subset to the emoji planes (1411 glyphs, 747 KB). Reproducible via
  scripts/build_emoji_subset.py. Embedded via INCBIN like the CJK subset.
- Typography::loadFont merges it (MergeMode) only into the small text fonts
  (Body/Subtitle/Caption/Button) — not headers, which don't need 1400 emoji.
  The base font keeps precedence for U+2600–26FF, so text-style symbols stay.

Limits: ImGui does no shaping, so single-codepoint emoji render but ZWJ sequences
(family/profession) and regional-indicator flags won't compose; emoji are
monochrome (the OS emoji picker still inputs them fine, and the composer byte
counter already counts their 4-byte UTF-8 cost against the on-chain cap).

Verified headless: sizeof(ImWchar)==4 and every probed emoji is in-font and bakes
into the atlas.

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

scripts/build_emoji_subset.py

@@ -0,0 +1,87 @@
+#!/usr/bin/env python3
+"""
+Build a monochrome Noto Emoji subset for the chat/message UI.
+
+Dear ImGui rasterizes fonts with stb_truetype, which handles only monochrome
+(outline `glyf`) fonts — NOT color emoji (CBDT/sbix/COLR). So we use Google's
+*monochrome* Noto Emoji (github.com/google/fonts, ofl/notoemoji, OFL-licensed)
+and merge it into the text fonts (see Typography::loadFont, the block after the
+CJK merge). ImGui renders one glyph per codepoint with no shaping, so ZWJ
+sequences / regional-indicator flags won't compose — single-codepoint emoji
+(😀 🎉 ❤ 🔥 👍 …) render fine, which covers the overwhelming majority of use.
+
+Source is the variable font pinned to wght=400 → static, then subset to the
+emoji planes plus the higher symbol/star ranges the base UI font doesn't cover.
+The base Ubuntu font already owns U+2600–26FF etc.; ImGui's MergeMode gives the
+first-loaded glyph precedence, so those stay text-styled and only the codepoints
+the base lacks fall through to this font.
+
+Get the source once (OFL, redistributable):
+  curl -fsSL -o /tmp/NotoEmoji-VF.ttf \
+    'https://github.com/google/fonts/raw/main/ofl/notoemoji/NotoEmoji%5Bwght%5D.ttf'
+
+Then: python3 scripts/build_emoji_subset.py
+Output: res/fonts/NotoEmoji-Subset.ttf  (committed; embedded via INCBIN)
+"""
+import os
+from fontTools import ttLib, subset
+from fontTools.varLib.instancer import instantiateVariableFont
+
+ROOT = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+SOURCE_VF = '/tmp/NotoEmoji-VF.ttf'
+STATIC = '/tmp/NotoEmoji-Static.ttf'
+OUTPUT = os.path.join(ROOT, 'res', 'fonts', 'NotoEmoji-Subset.ttf')
+
+# Emoji codepoints to keep. Ranges are inclusive.
+RANGES = [
+    (0x1F000, 0x1FAFF),  # all the main emoji planes (emoticons, pictographs, transport, supplement, extended)
+    (0x2600, 0x27BF),    # Miscellaneous Symbols + Dingbats
+    (0x2B00, 0x2BFF),    # stars (⭐ 2B50) and misc arrows
+    (0xFE00, 0xFE0F),    # variation selectors (VS16 emoji-style)
+    (0x2194, 0x21AA),    # arrows used as emoji
+    (0x231A, 0x231B),    # ⌚ ⌛
+    (0x23E9, 0x23FA),    # media-control emoji
+    (0x25AA, 0x25FE),    # small squares
+]
+SINGLES = [0x200D, 0x2934, 0x2935, 0x3030, 0x303D, 0x3297, 0x3299,
+           0x00A9, 0x00AE, 0x2122, 0x2139, 0x24C2]
+
+
+def main():
+    if not os.path.exists(SOURCE_VF):
+        raise SystemExit(f"missing source font {SOURCE_VF} — see the header for the curl command")
+
+    # 1. Pin the weight axis so stb_truetype rasterizes a clean static instance.
+    f = ttLib.TTFont(SOURCE_VF)
+    if 'fvar' in f:
+        instantiateVariableFont(f, {'wght': 400}, inplace=True)
+    f.save(STATIC)
+
+    unicodes = list(SINGLES)
+    for lo, hi in RANGES:
+        unicodes.extend(range(lo, hi + 1))
+
+    opts = subset.Options()
+    opts.layout_features = []          # ImGui does no shaping — drop GSUB/GPOS
+    opts.name_IDs = []
+    opts.notdef_outline = True
+    opts.glyph_names = False
+    opts.drop_tables = ['GSUB', 'GPOS', 'GDEF', 'morx', 'kern']
+
+    font = subset.load_font(STATIC, opts)
+    ss = subset.Subsetter(options=opts)
+    ss.populate(unicodes=unicodes)
+    ss.subset(font)
+    subset.save_font(font, OUTPUT, opts)
+
+    out = ttLib.TTFont(OUTPUT)
+    cmap = out.getBestCmap()
+    color = any(t in out.reader.keys() for t in ('CBDT', 'sbix', 'COLR'))
+    print(f"Output: {OUTPUT}")
+    print(f"Size: {os.path.getsize(OUTPUT)//1024} KB | glyphs: {len(cmap)} | color tables: {color}")
+    if color:
+        raise SystemExit("ERROR: subset has color tables — stb_truetype cannot render it")
+
+
+if __name__ == '__main__':
+    main()