Feature: translations, configurable casing, and search box alignment

[Piero-93]

Summary

Adds internationalization (i18n) and two appearance options to nirilayout.

• Translations — all interface text is now localizable through gettext (.po/.mo, via leonelquinteros/gotext), with catalogs embedded into the binary via go:embed. English is the source language; Italian is included as the first translation.
• Language selection — chosen automatically, with this precedence:
  1. the -lang flag (e.g. -lang it) overrides everything;
  2. otherwise the OS locale, from LC_ALL → LC_MESSAGES → LANG (it_IT.UTF-8 is matched to its base language it);
  3. otherwise English.
• Casing — interface text now uses standard sentence casing. The new -lowercase flag renders all of nirilayout's own text in lowercase. It only affects nirilayout's interface — your layout and output names are shown exactly as written.
• Search box alignment — the new -leftalign flag left-aligns the search box text (centered by default).

Command-line options added

Option	Description
-lang <code>	Interface language code (e.g. it); overrides the locale
-lowercase	Render nirilayout's interface text in lowercase
-leftalign	Left-align the text in the search box

For maintainers

Compiled .mo files are committed and embedded, so building or running nirilayout never requires gettext tooling — only editing translations does.
New Makefile targets:

• make pot — regenerate the .pot template from the sources
• make update-po — merge the template into existing .po files
• make i18n — refresh .po files and recompile .mo

Adding a language is documented in the README (Localization → Adding a translation).

Docs

README gains Command-line options, Appearance (Casing and Search box alignment), and Localization sections, plus a contributor guide for translations.

Testing

• go vet ./... — clean
• go test ./... — passing (i18n_test.go covers language normalization, English fallback, -lowercase, and the Italian catalog)
• Verified --help end to end in English and Italian, and -lowercase