Improve Bluetooth error reporting with user-friendly messages

This is the continuation of the feature I started in MR !231 (closed). This change improves how connection errors are presented to users across BlueDevil. Instead of exposing raw BlueZ D-Bus error identifiers directly, errors are mapped to clear, localized, user-friendly messages.

High-level design (before):

The kcm and applet ui exposes the raw BlueZ error string.

flowchart LR
    subgraph Before["Before"]
        UI1["KCM / Applet UI"]
        BQ1["BluezQt::PendingCall"]
        UI1 -->|"1. errorText()"| BQ1
        BQ1 -->|"2. raw BlueZ error string"| UI1
        UI1 -->|"3. display"| User1["User sees raw error\n(e.g. br-connection-timeout)"]
    end

High-level design (after):

Error-to-text mapping is centralized in the BlueDevil kded daemon. UI components delegate error interpretation to the daemon via D-Bus, ensuring consistent messaging across the KCM and the applet. However, I am not certain this is the optimal architectural solution, and I would appreciate feedback on alternative designs.

flowchart LR
    subgraph After["After"]
        UI2["KCM / Applet UI"]
        BQ2["BluezQt::PendingCall"]
        DBus["D-Bus: org.kde.BlueDevil\ngetErrorMessage()"]
        Daemon["BlueDevil kded\nError mapping + i18n"]

        UI2 -->|"1. errorText()"| BQ2
        BQ2 -->|"2. raw error"| UI2
        UI2 -->|"3. query"| DBus
        DBus --> Daemon
        Daemon -->|"4. user-friendly text"| DBus
        DBus --> UI2
        UI2 -->|"5. display"| User2["User sees friendly message"]
    end

Error message:

Before (current behavior)	After (with this change)

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BUG: 467122 BUG: 475909