fix: support a non-English base language in localize & generate (#11)

[mike-doctorina]

Fixes #11.

The column layout is label | description | meta | <source> | <locale> .... The fourth (source) column was assumed to be English, so a sheet whose base language was ru, de, etc. was mishandled. This affected both executables — they are separate code paths, so both are fixed.

localize (lib/src/localize/)

Previously isLocalizationHeader only accepted an English fourth column, so a ru source was rejected:

Sheet "auth" is not a localization sheet (expected "label | description | meta | en | <locale> ..." header, got [label, description, meta, ru]), skipping sheet

• isLocalizationHeader now accepts any header whose fourth column resolves to a recognized language (resolveLanguageName != null). Non-language data columns (reference tables, e.g. a Family column) are still rejected, so those sheets are never overwritten.
• The source language code is captured from the header and threaded through LocalizeRow.sourceCode into buildLocalizationPrompt, which now emits source_language: / source_text: and names the actual source language in the prompt and JSON schema instead of hard-coding English. Defaults to English, so existing sheets are unchanged.

generate (bin/generate.dart)

generateLocalizationTable already treated columns 3+ as ordinary locales (no English requirement), but generateFlutterLocalization hard-coded the flutter gen-l10n template to <prefix>_en.arb, so a bucket without an English column failed to generate.

• New selectTemplateArb chooses the template per bucket: English when present (unchanged behaviour), otherwise the first available ARB — so a non-English base language generates too.

Release / hygiene

• Version bumped 0.5.0 → 0.6.0; both fixes recorded in CHANGELOG.md.
• Added CLAUDE.md documenting the release-hygiene requirement every task must satisfy (version bump + CHANGELOG + dart pub publish --dry-run).
• dart pub publish --dry-run → 0 warnings.

Tests

• isLocalizationHeader accepts ru / pt-BR sources; still rejects a non-language fourth column.
• extractEmptyCells carries a non-English sourceCode (incl. normalizing pt-BR → pt_BR) and reads the source text from the fourth column.
• buildLocalizationPrompt names a non-English source in the prompt and schema, and still defaults to English.
• selectTemplateArb prefers English, falls back for a non-English base, honours a custom prefix.
• Full suite: 109 passing, dart analyze clean.

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