[ernolf]

Summary

Adds a default quota for new guest accounts, configurable from the Guests tab.

This is the second part of #1602, which was split in two: the allowlist regression fix landed separately as #1616, and this PR carries the quota feature.

Feature

• Quota selector on the Guests tab, mirroring the Accounts quota field: Default (…), Unlimited, 1 GB / 5 GB / 10 GB, or a typed custom value. Typing a number offers it with selectable units (e.g. 500 becomes 500 KB / 500 MB / 500 GB); a unit is required so the stored value is unambiguous.
• The Default option shows the actual preset-derived value (e.g. "Default (1 GB)"), read from the config lexicon via IAppConfig::getKeyDetails(). Selecting it removes the override so the preset default applies again, and the label follows the active Quick preset even while an override is set.

Backend: Config gains getGuestQuota(), hasGuestQuotaOverride(), getGuestQuotaDefault() and setGuestQuota(); SettingsController::getConfig() returns guestQuota (the default sentinel when no override is stored) and guestQuotaDefault, and setConfig() accepts and validates guestQuota. GuestManager already applies guest_quota on guest creation, so guest behaviour is unchanged.

Docs

Documents the default guest quota in the README and corrects three links that still pointed to the removed master branch.

Closes #1601

Testing

• Pick a preset, Unlimited, or a typed size, reload: the value persists.
• "Default (…)" removes the override; a bare number without a unit is rejected.
• Switching the Quick preset updates the "Default (…)" label, including when an override is currently stored.

🤖 AI (if applicable)

• The content of this PR was partly generated using AI
  Assisted-by: ClaudeCode:claude-opus-4-8

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[pringelmann]

Thank you, lgtm!

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