feat(queue): add empty_message + empty_filter_message override

[foXaCe]

Summary

Adds two optional fields on ap_queue_opts so callers can override the built-in NO ITEMS IN QUEUE. / NO ITEMS MATCH THIS FILTER. placeholders without forking the widget.

typedef struct {
    // ...
    const char *empty_message;        // overrides "NO ITEMS IN QUEUE."
    const char *empty_filter_message; // overrides "NO ITEMS MATCH THIS FILTER."
} ap_queue_opts;

When the override is NULL or empty, the existing English strings are used — fully backwards compatible.

Why

I'm wiring NextUI's i18n layer into ScrapeGoat (companion PR: https://github.com/Helaas/nextui-scrapegoat-pak) and the queue's empty-state placeholders were the only Apostrophe-rendered strings I couldn't override from the calling pak. Without this change downstream apps either show an English string in an otherwise localised UI, or have to ship a forked copy of apostrophe_widgets.h just to swap two literals.

Diff

const char *msg;
if (stat_total == 0)
    msg = (opts->empty_message && opts->empty_message[0])
          ? opts->empty_message : "NO ITEMS IN QUEUE.";
else
    msg = (opts->empty_filter_message && opts->empty_filter_message[0])
          ? opts->empty_filter_message : "NO ITEMS MATCH THIS FILTER.";

Same shape as the existing filter_labels[4] override added earlier.

Test plan

• Existing callers (with both fields unset) render the original English placeholders.
• When overrides are set, the localised strings show up in both the all-empty and filter-empty cases.
• Built and ran on a TrimUI Brick (tg5040) via ScrapeGoat with language=fr in minuisettings.txt.

[Copilot]

Pull request overview

Adds optional overrides to the queue viewer so downstream apps can localize the empty-state placeholder strings without forking the widget, while keeping default behavior when overrides are unset.

Changes:

• Extend ap_queue_opts with empty_message and empty_filter_message optional fields.
• Update ap_queue_viewer empty-state rendering to prefer these overrides when non-NULL and non-empty.

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[Helaas]

Thanks for the PR. i18n and the other PRs are currently being discussed on the Discord.
It'll be all or nothing I guess.

[foXaCe]

I did the OS and the store too

LoveRetro/nextui-pak-store#136
LoveRetro/NextUI#735