feat(contacts): animated avatars (GIF/WebP) with a Settings toggle

Animate contact avatars end to end:

- texture_loader gains LoadAnimatedRGBA: decodes an image into a downscaled
  RGBA frame sequence + per-frame durations — animated GIF via stb
  (stbi_load_gif_from_memory) and animated WebP via libwebp's WebPAnimDecoder;
  stills (and APNG, which stb reads as one image) return a single frame. Frames
  are box-downscaled (smaller cap for animations) to bound VRAM, capped at 300.
- The contacts avatar cache now holds a frame sequence; currentAvatarFrame()
  advances animated avatars by the ImGui clock and is used everywhere avatars
  draw (list, cards, table, grid, preview). When a live animated frame is drawn
  it flags the render loop (ConsumeContactsAvatarAnimation, clear-on-read) so
  main.cpp keeps producing frames while animation plays and idles when it stops
  or the contacts view is hidden.
- New animate_avatars setting (default on) + a Settings appearance toggle
  ("Animate avatars"); off shows the first frame only. currentAvatarFrame
  honors it. +i18n (8 langs, CJK subset rebuilt for 帧/播/첫).

Verified: a 3-frame GIF and a 3-frame animated WebP both decode to 3 frames
with correct 120ms delays through the exact libwebp/stb calls used here.

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

src/util/i18n.cpp

@@ -389,6 +389,7 @@ void I18n::loadBuiltinEnglish()
         strings_["simple_background"] = "Simple background";
         strings_["console_scanline"] = "Console scanline";
         strings_["theme_effects"] = "Theme effects";
+        strings_["animate_avatars"] = "Animate avatars";
         strings_["acrylic"] = "Acrylic";
         strings_["noise"] = "Noise";
         strings_["ui_opacity"] = "UI Opacity";
@@ -613,6 +614,7 @@ void I18n::loadBuiltinEnglish()
         strings_["tt_simple_bg_alt"] = "Use a gradient version of the theme background image\nHotkey: Ctrl+Up";
         strings_["tt_scanline"] = "CRT scanline effect in console";
         strings_["tt_theme_effects"] = "Shimmer, glow, hue-cycling per theme";
+        strings_["tt_animate_avatars"] = "Play animated (GIF / WebP) contact avatars; off shows the first frame only";
         strings_["tt_blur"] = "Blur amount (0%% = off, 100%% = maximum)";
         strings_["tt_noise"] = "Grain texture intensity (0%% = off, 100%% = maximum)";
         strings_["tt_ui_opacity"] = "Card and sidebar opacity (100%% = fully opaque, lower = more see-through)";