Remove legacy FTL extract and stub helpers

[andrew000]

Summary

• Removed legacy FTL extract CLI modules.
• Removed the old Fluent stub helper and .ftl support from i18n stub.
• Kept i18n stub focused on gettext inputs: .po, .pot, and .mo.
• Rewrote the README with clearer installation, backend, quick start, CLI, and FTL-Extract guidance.

Breaking Change

FTL extraction and FTL stub generation are no longer provided by aiogram_i18n.
Use FTL-Extract for those workflows instead.

Testing

• just lint

[Copilot]

Pull request overview

This PR removes legacy Fluent (FTL) extraction and stub-generation helpers from aiogram_i18n, keeps the i18n stub CLI command focused on GNU gettext catalogs, and refreshes the README to reflect the new guidance (FTL workflows moved to FTL-Extract).

Changes:

• Removed deprecated Fluent extraction CLI modules and the Fluent stub helper.
• Updated i18n stub to support only .mo, .po, and .pot inputs and adjusted CLI behavior accordingly.
• Rewrote the README with updated installation/backends/CLI documentation and FTL-Extract guidance.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/aiogram_i18n/utils/fluent_stub/__init__.py	Removes the legacy Fluent stub helper module.
src/aiogram_i18n/utils/cli/stub.py	Drops .ftl support and adjusts stub command behavior to gettext inputs only.
src/aiogram_i18n/utils/cli/multiple_extract.py	Removes the legacy “multiple extract” Fluent CLI module.
src/aiogram_i18n/utils/cli/extract.py	Removes the legacy Fluent extract CLI module.
src/aiogram_i18n/utils/cli/__init__.py	Stops exporting/registering the removed extract commands.
README.md	Updates documentation and guidance, including directing FTL workflows to FTL-Extract.

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[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

src/aiogram_i18n/utils/cli/stub.py:40

• The validation error "only files allowed" is too vague for CLI users (it doesn't indicate which path failed or that the path must exist and be a file). Consider including the invalid path and a clearer description (e.g., "input path must be an existing file").

        if not path.is_file():
            msg = "only files allowed"
            raise click.ClickException(msg)

[andrew000]

@RootShinobi review