Validate Mbin community names when creating a community

[neo22neo]

Problem

When creating a community on an Mbin server, the form only disabled the submit button on an empty name. Mbin rejects any magazine name outside /^[a-zA-Z0-9_]{2,25}$/ (RegPatterns::MAGAZINE_NAME), so names with spaces, hyphens, accents, etc. failed server-side with no guidance for the user.

Changes

• lib/src/utils/mbin_community_name.dart (new) — pure, testable helpers:
  • isValidMbinCommunityName — the ^[a-zA-Z0-9_]{2,25}$ check.
  • mbinCommunityNameIssue — why a name is invalid (invalidCharacters / tooShort / tooLong); empty is treated as "not entered yet".
  • suggestMbinCommunityName — best-effort sanitized name (unsupported runs → _, collapsed, trimmed, truncated to 25), or null when nothing usable can be salvaged.
• lib/src/widgets/text_editor.dart — shared TextEditor gains helperText / errorText passthrough.
• lib/src/screens/explore/community_owner_panel.dart — in CommunityOwnerPanelGeneral (creation path only), when the active server is Mbin:
  • show the naming rule as helper text,
  • show an inline error while the name is invalid,
  • offer a one-tap Use “…” action to apply the suggested name,
  • keep the submit button disabled until the name is valid.
  • Lemmy / PieFed behaviour is unchanged (still maxLength: 25, no extra validation).
• lib/l10n/app_en.arb — new keys: community_nameMbinHelp, community_nameInvalidCharacters, community_nameTooShort, community_nameTooLong, community_nameUseSuggestion.
• pubspec.yaml / test/ — add flutter_test dev dependency and focused unit tests for the new helpers.

Notes

• The regex matches upstream Mbin (MbinOrg/mbin src/Utils/RegPatterns.php).
• I don't have a Flutter toolchain in this environment, so dart format / flutter test / flutter gen-l10n were not run locally — please double-check CI.

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