Add CLI arguments and environment variables to customize all login page elements:

- Login title, subtitle, and welcome text
- Password field placeholder and submit button text
- Password instruction messages (config file, env var, hashed)
- Error messages (rate limit, missing/incorrect password)

New CLI options:
  --login-title, --login-below, --password-placeholder, --submit-text
  --login-password-msg, --login-env-password-msg, --login-hashed-password-msg
  --login-rate-limit-msg, --missing-password-msg, --incorrect-password-msg

New environment variables:
  CS_LOGIN_TITLE, CS_LOGIN_BELOW, CS_PASSWORD_PLACEHOLDER, CS_SUBMIT_TEXT
  CS_LOGIN_PASSWORD_MSG, CS_LOGIN_ENV_PASSWORD_MSG, CS_LOGIN_HASHED_PASSWORD_MSG
  CS_LOGIN_RATE_LIMIT_MSG, CS_MISSING_PASSWORD_MSG, CS_INCORRECT_PASSWORD_MSG

Features:
- Full backwards compatibility with existing --app-name/--welcome-text
- HTML escaping for security (prevents XSS)
- Config file support (YAML)
- Priority: CLI args > env vars > config file > defaults
- Internationalization preserved for non-customized messages

Perfect for Docker deployments and corporate branding.

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Co-Authored-By: Claude <noreply@anthropic.com>

Replace non-scalable individual flags (--login-title, --login-below, etc.)
with a unified --custom-strings flag that accepts JSON file paths or inline
JSON for UI customization. This leverages the existing i18n system for
better scalability and maintainability.

Changes:
- Add --custom-strings flag with JSON validation
- Extend i18n system to merge custom strings with defaults
- Remove newly-added individual login/UI flags
- Deprecate legacy --app-name and --welcome-text flags
- Update login route to use unified i18n system
- Add comprehensive tests for new functionality
- Update documentation with migration guide and examples

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Co-Authored-By: Claude <noreply@anthropic.com>

Remove migration examples for flags that were never released.
Only --app-name and --welcome-text were in the original codebase
and might be used by existing users.

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Co-Authored-By: Claude <noreply@anthropic.com>

Replace outdated CS_* environment variable examples with the new
--custom-strings flag approach. Include both inline JSON and
mounted file examples for Docker users.

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Co-Authored-By: Claude <noreply@anthropic.com>

Remove login page customization sections from FAQ.md and install.md as they were too specific for those documents. Move customization reference to guide.md where it's more appropriate.

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Co-Authored-By: Claude <noreply@anthropic.com>

Remove duplicate validation in cli.ts since comprehensive validation already exists in loadCustomStrings(). Also eliminate unnecessary customStrings global variable by using customStringsData directly.

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Co-Authored-By: Claude <noreply@anthropic.com>

…dling

Make customStringsArg parameter required since caller already checks existence. Remove redundant try-catch block in main.ts to avoid duplicate error logging since loadCustomStrings already provides descriptive error messages.

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Co-Authored-By: Claude <noreply@anthropic.com>

Remove tests for legacy individual flags (--login-title, --password-placeholder, --submit-text, etc.) that have been replaced by the unified --custom-strings flag. Also remove cleanup of non-existent environment variables in CLI tests.

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Co-Authored-By: Claude <noreply@anthropic.com>

Replace manual resource merging and re-initialization with i18next's built-in addResourceBundle API. This is more efficient, cleaner, and the idiomatic way to add custom translations dynamically.

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Co-Authored-By: Claude <noreply@anthropic.com>

Change "custom UI strings" to "custom translations" to better reflect that this feature uses the i18n system and could be used for more than just UI elements, making it more accurate and future-proof.

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Co-Authored-By: Claude <noreply@anthropic.com>

Remove deprecation from --app-name since it serves a valuable purpose for the {{app}} placeholder in custom strings, especially useful for internationalization. Update documentation to show how --app-name works with --custom-strings and clarify that only --welcome-text is deprecated.

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Co-Authored-By: Claude <noreply@anthropic.com>

Change CLI flag from --custom-strings to --i18n to use standard internationalization terminology. This is more accurate since the feature leverages the i18next system and follows industry conventions. Update all documentation, tests, and code references.

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Co-Authored-By: Claude <noreply@anthropic.com>

Replace login-specific customization.md with generic internationalization section in guide.md. This approach is more maintainable, reduces file sprawl, points to source files as truth, and encourages community contributions. Removes repetitive examples that could get out of sync.

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Co-Authored-By: Claude <noreply@anthropic.com>