feat(osd): add privacy indicator OSD when mic, camera, screenshare enable/disable

[mattvilim]

Summary

#3040 added a privacy indicator widget to show mic, camera, screenshare state. This PR adds OSD notifications when privacy indicator enables/disables.

This PR adds 6 i18n OSD keys:

• osd.privacy.mic-on
• osd.privacy.mic-off
• osd.privacy.camera-on
• osd.privacy.camera-off
• osd.privacy.screen-on
• osd.privacy.screen-off

It adds 2 i18n UI keys:

• settings.schema.shell.osd-kinds-privacy.label
• settings.schema.shell.osd-kinds-privacy.description

The new OSD config to toggle this feature is added to GUI settings. If the PR is accepted, I will create a docs PR to update the OSD config documentation.

Motivation

Gives more visible notice of privacy state changes

Type of Change

• Bug fix
• New feature
• Breaking change
• Refactoring
• Build / packaging

Related Issue

#3040

Testing

I tested with mic and screen share enable/disable. I don't have a camera and did not test it. When multiple privacy events happen in rapid succession (e.g. leave call that had screen share and mic enabled), the existing OSD popup logic handles animating the first and showing the others overwritten in succession.

Manual Coverage

• Tested on Niri
• Tested on Hyprland
• Tested on Sway
• Tested on another compositor:
• Tested with different bar positions and density settings
• Tested at different interface scaling values
• Tested with multiple monitors

Screenshots / Videos

Checklist

• This PR is ready for review, or it is marked as Draft.
• I read and followed the relevant guidance in CONTRIBUTING.md.
• I ran just format with clang-format v22+ installed, or this PR has no code changes.
• I ran the relevant build or test commands, or explained why they were not run.
• I self-reviewed the changes.
• I checked for new warnings or errors.
• I will update end-user documentation after merge, or this PR does not change user-facing configuration or behavior.
• I added or updated assets/translations/en.json, or this PR adds no new user-facing strings.
• I did not edit non-English translation files unless this PR is explicitly for translation tooling, an import/export sync, or a maintainer-requested locale change.
• I used the existing canonical names for config keys, IPC names, paths, and identifiers.

Additional Notes

[mattvilim]

../src/shell/desktop/widgets/desktop_button_widget.cpp:29:20: error: std::move of the variable 'labelColor' of the trivially-copyable type 'std::optional<ColorSpec>' has no effect; remove std::move() [performance-move-const-arg,-warnings-as-errors]
   29 |       m_labelColor(std::move(labelColor)), m_hoverBackground(std::move(hoverBackground)),
      |                    ^~~~~~~~~~          ~
../src/shell/desktop/widgets/desktop_button_widget.cpp:29:62: error: std::move of the variable 'hoverBackground' of the trivially-copyable type 'ColorSpec' has no effect; remove std::move() [performance-move-const-arg,-warnings-as-errors]
   29 |       m_labelColor(std::move(labelColor)), m_hoverBackground(std::move(hoverBackground)),
      |                                                              ^~~~~~~~~~               ~

Lint failure is in main and unrelated to this PR.

[ItsLemmy]

thanks