[FEATURE] GNOME Online Accounts support for calendar settings

[bennypowers]

Submission checklist

• I have searched existing issues and confirmed this has not been requested before.
• I have checked existing pull requests for similar changes.

Feature type

Integration support

Feature summary

Allow calendar settings to pull in user calendars from GNOME online accounts

Motivation / use case

Parity with v4, integration with common user patterns

Proposed solution

something along the lines of:

// calendar_service.h — new members
std::unique_ptr<GoaBackend> m_goaBackend;
std::vector<GoaBackend::GoaAccount> m_goaAccounts;

In tick():

• For each GOA account with isOAuth2: call GetAccessToken(), use result as bearer in CalDavClient (or GoogleClient for Google, bypasses api.noctalia.dev entirely)
• For password-based GOA: call GetPassword(), feed into CalDavClient with calDavUri

// New module: src/calendar/goa_backend.h/.cpp

class GoaBackend {
public:
    struct GoaAccount {
        std::string objectPath;   // /org/.../account_1234
        std::string displayName;
        std::string providerName; // "google", "nextcloud", etc.
        std::string calDavUri;    // from Calendar.Uri
        bool isOAuth2;
    };

    void start(TickCallback onChange);
    std::vector<GoaAccount> accounts() const;

    // Called before each sync — returns fresh token or password
    std::variant<std::string /*bearer*/, std::string /*password*/>
        getCredential(const GoaAccount&);
};

No config changes, GOA accounts are auto-detected and live beside manual accounts

Alternatives considered

• manually add accounts (duplicative, time consuming)
• khal

References / related projects

No response

Additional context

No response