Add discord rpc

[ThePotato97]

closes #186

Add Discord Rich Presence (RPC) Support

This adds basic support for Discord Rich Presence to psst.

If you enable it and provide a valid Discord Application ID, the app will connect to your running Discord client and show the currently playing track in your profile.

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Changes

• Added a new discord_rpc.rs module under psst-core to manage the Discord RPC connection in a background thread.
• Added discord-presence as a new dependency.
• Extended the user configuration with two new fields:
  • discord_rpc_enable (bool)
  • discord_rpc_app_id (string)
• Updated the Preferences UI with:
  • A checkbox to enable/disable Discord RPC.
  • A text field for entering your Discord App ID.
  • A button linking to Discord Developer Portal if you need to create one.
• Hooked the RPC updates into the PlaybackController:
  • Starts the RPC client when enabled (and app ID is valid).
  • Updates it live if the App ID changes.
  • Shuts it down cleanly when disabled or when quitting.

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Behavior

• Nothing happens unless the user explicitly enables the feature.
• Invalid App IDs are rejected cleanly with warnings.
• If playback changes (play, pause, seek, next track), the Discord status updates automatically.
• Disconnects and clears the Discord status when pausing or stopping playback.

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Notes

• Only one new real dependency was added: discord-presence.
• Defaults are safe: Discord RPC is disabled by default.

Screenshots

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Why

Because Discord status is cool
and now people can see what garbage music you're listening to in real time.

[jacksongoode]

This is nice - thank you for contributing! but I'd like to do a proper re-review after the unrelated linting changes were resolved.

[SO9010]

I had a little skim over this PR! It's looking good so far!

Also I think the taplo.toml shouldn't be included to the PR.

[SO9010]

Just a question, do we have to make it so that the user makes up their ID or are we able to say generate it for them so its more automatic?

[Cleboost]

Link #186 to close