feat(chat): emoji picker + show-hidden toggle

- Emoji picker: a 🙂 button on the composer row opens a scrollable grid of ~150
  common single-codepoint emoji (all verified present in the bundled NotoEmoji
  subset); clicking appends to the composer, respecting the byte cap. ImGui does
  no shaping, so the set is single-codepoint only (ZWJ sequences / flags omitted).
- Show hidden: when there are hidden conversations, a "Show hidden (N)" toggle
  appears in the list; toggling it reveals them (dimmed) and the thread header's
  Hide button becomes Unhide. Off by default, and resets when nothing is hidden.

8-language strings; no new CJK glyphs.

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

src/util/i18n.cpp

@@ -253,6 +253,9 @@ void I18n::loadBuiltinEnglish()
         strings_["chat_mute"] = "Mute";
         strings_["chat_unmute"] = "Unmute";
         strings_["chat_hide"] = "Hide";
+        strings_["chat_unhide"] = "Unhide";
+        strings_["chat_show_hidden"] = "Show hidden";
+        strings_["chat_hide_hidden"] = "Hide hidden";
         strings_["chat_hidden_toast"] = "Conversation hidden — a new message brings it back";
         strings_["chat_pick_contact"] = "Choose from contacts\xE2\x80\xA6";
         strings_["chat_no_z_contacts"] = "No shielded-address contacts yet";