feat(locale): support nested key references in phrase interpolation

[Kenshiin13]

Description

Locale files can already contain nested objects, and locale('parent.child') resolves them correctly because flattenDict runs at load time. Phrase interpolation with ${...} is the only place this breaks down: the resolver looks names up in the un-flattened nested table, so ${parent.child} silently fails to substitute even though the same key is reachable via a direct locale() call.

for k, v in pairs(flattenDict(locales, {})) do        -- iterates flat keys
    if type(v) == 'string' then
        for var in v:gmatch('${[%w%s%p]-}') do
            local locale = locales[var:sub(3, -2)]    -- looks up un-flattened tree

This patch threads the already-flattened dict through the resolver so both lookup paths behave the same. Top-level references keep working; nested references start working as a side effect.

Example

locales/en.json:

{
  "greeting": "Hello",
  "user": { "name": "Kenshin13" },
  "msg": "${greeting}, ${user.name}!"
}

print(locale('msg'))
-- before: Hello, ${user.name}!
-- after:  Hello, Kenshin13!