Add automatic MPD server management

[htkhiem]

This implements #339 and will allow Euphonica to become a "music player" in the traditional sense: you install the package/Flathub app, point it to a folder with music, and hit play. All the "MPD stuff" like setting up a config file, starting and stopping the server shall be handled transparently by Euphonica.

In this mode, Euphonica only borrows the MPD executable. It does not hijack control of any existing MPD server running on your system; rather, it spins up its own instance with a socket connection to avoid collisions and an autogenerated config file stored in the config folder of Euphonica, not MPD, again to avoid collisions with existing configs.

Configuration

Now that we're managing our own MPD server, that means the burden of writing the config file is upon us. To this end Euphonica will feature a GUI to configure certain fields that still make sense in a local-player context, e.g. we'll mostly focus on audio outputs and where the library is; things like where the DB files, sticker file, MPD playlist folders should be won't be exposed to the user.

Sneak peek of output config page (multiple outputs can be configured):

Progress:

• Subprocess management (start/stop/restart)
• Basic config file parsing and writing
• Basic audio output plugin config (just the common settings)
• Hidden managed FIFO output for visualiser
• Force visualiser into FIFO mode & to read from managed path when in standalone mode
• Fix janky FIFO visualiser behaviour until it looks like PipeWire side
• Programmatically construct plugin-specific config keys with suitable ActionRow subclasses. Configuring these is mandatory for many plugins, like user-configured FIFO.
• Hide managed FIFO output from bottom bar/pane.
• Allow customising MPD binary path + add MPD path autodetection for things like Flatpak + add more toplevel config such as resamplers.
• Add onboarding pages to let users pick between Standalone Mode ("Use Euphonica like a music player") and client mode ("Connect Euphonica to an MPD-compatible server") + subsequent setup pages, like setting library path or connection address.
• Update AUR and Flatpak distributions to now include mpd as a hard dependency (+ maybe some resampler libs too).
• Maybe add a new AUR package without MPD deps and with standalone mode hidden from config (euphonica-client-git) should some want to keep their system svelte.
• Rewrite ALL them READMEs to re-frame ourselves as a "music player" instead.
• Maybe add a new wiki page to document the whole thing?

[aaravrav]

• todo for me after this PR is merged: add a nixos module for euphonica to automatically manage mpd