feat(dictionary): add Hachidori backend support

- Add backend selection, setup gating, Anki integration, and external host support
- Add launcher flags, documentation, packaging, and focused tests
- Open on-demand overlay modals on the first attempt

vendor/hachidori/wasm/env.sh

@@ -0,0 +1,25 @@
+#!/bin/sh
+# Shared Emscripten environment for every build entry point in this repo.
+#
+# emsdk's launchers run `python3` from PATH and require >= 3.10, but this host's
+# /usr/bin/python3 is 3.9. EMSHIM puts a newer python3 first on PATH.
+EMSDK_DIR="${EMSDK_DIR:-$HOME/emsdk}"
+EMSHIM="${EMSHIM:-$HOME/.local/emshim}"
+
+if [ ! -x "$EMSHIM/python3" ]; then
+  mkdir -p "$EMSHIM"
+  for v in 3.13 3.12 3.11 3.10; do
+    if [ -x "$HOME/.local/bin/python$v" ]; then
+      ln -sf "$HOME/.local/bin/python$v" "$EMSHIM/python3"
+      break
+    fi
+  done
+fi
+
+PATH="$EMSHIM:$PATH"
+export PATH
+export EMSDK_PYTHON="$EMSHIM/python3"
+# shellcheck disable=SC1091
+. "$EMSDK_DIR/emsdk_env.sh" >/dev/null 2>&1
+PATH="$EMSHIM:$PATH"
+export PATH