Audacity 4 uses messy app config directory layout

### Problem to be solved

I looked for Audacity's config files (since I had to wipe them to resolve issue #11258 for myself), noticing how they're laid out:

```
❯ ls -l ~/.config/{Audacity,audacity,audacity4}
/home/p0358/.config/audacity:
-rw-r--r-- 1 p0358 p0358 8071 lip 17 01:15 audacity.cfg
-rw-r--r-- 1 p0358 p0358 52333 lip 16 23:54 pluginregistry.cfg
-rw-r--r-- 1 p0358 p0358 26 paź 4 2025 pluginsettings.cfg

/home/p0358/.config/Audacity:
-rw-r--r-- 1 p0358 p0358 805 sty 22 2026 Audacity4Development.ini
-rw-r--r-- 1 p0358 p0358 1739 wrz 3 20:01 Audacity4.ini

/home/p0358/.config/audacity4:
-rw-r--r-- 1 p0358 p0358 65821 sty 22 2026 pluginregistry.cfg
-rw-r--r-- 1 p0358 p0358 89 sty 22 2026 pluginsettings.cfg
```

There's now a duplicated `audacity` and `Audacity` folders in .config, respectively belonging to Audacity 3 and 4 versions. Furthermore, there's `audacity4` as well, further adding to the confusion.

After wiping all of these and launching only Audacity 4 anew, we see:
```
❯ ls -l ~/.config/{Audacity,audacity,audacity4}
ls: cannot access '/home/p0358/.config/audacity': No such file or directory
/home/p0358/.config/Audacity:
-rw-r--r-- 1 p0358 p0358 664 wrz 3 20:17 Audacity4.ini

/home/p0358/.config/audacity4:
-rw-r--r-- 1 p0358 p0358 61440 wrz 3 20:16 audiocom_sync.db
-rw-r--r-- 1 p0358 p0358 89 wrz 3 20:16 pluginsettings.cfg
```

This means that for some reason, Audacity 4 splits its config files into two directories, even though Audacity 3 didn't do so, which I consider to be a regression (perhaps and oversight).

### Your idea

For brevity and to avoid making a mess in config files, the app should put all its files in one directory, rather than splitting it into two. The capitalized folder name `Audacity` shouldn't be used at all, as it creates a confusion with lower-case `audacity` folder that might exist at the same time.

**The simple and clean solution is to simply keep Audacity 3 files in its existing `~/.config/audacity`, while putting Audacity 4 files in `~/.config/audacity4` (both ini and cfg).**

The config file `Audacity4.ini` should also be **lower-cased**, just like original `audacity.cfg` (it's really better to avoid capital letters in your config files, remember that Windows is the only system that defaults to case-agnostic filesystem, but on Linux and macOS `Audacity` and `audacity` are completely separate from each other, so not sticking to a convention can lead to issues later on).

So basically you should probably make it look like this (for Audacity 3 and 4 respectively):

```
❯ ls -l ~/.config/{audacity,audacity4}
/home/p0358/.config/audacity:
-rw-r--r-- 1 p0358 p0358 8071 lip 17 01:15 audacity.cfg
-rw-r--r-- 1 p0358 p0358 52333 lip 16 23:54 pluginregistry.cfg
-rw-r--r-- 1 p0358 p0358 26 paź 4 2025 pluginsettings.cfg

/home/p0358/.config/audacity4:
-rw-r--r-- 1 p0358 p0358 664 wrz 3 20:17 audacity4.ini
-rw-r--r-- 1 p0358 p0358 61440 wrz 3 20:16 audiocom_sync.db
-rw-r--r-- 1 p0358 p0358 89 wrz 3 20:16 pluginsettings.cfg
```

### Prior art

Audacity 3 simply stored its config files in `~/.config/audacity` and all file names were lower-case.

### Additional context

Only Windows has case-agnostic filesystem. This mess should not be spread to other operating systems, as it's bound to cause problems down the line.

No source file or test is named. Start by locating Audacity 4's configuration-directory initialization and references to Audacity4.ini, then inspect how the existing Audacity 3 directory is selected. Done means Audacity 4 keeps its ini, cfg, and database files under ~/.config/audacity4 with lowercase naming, without using ~/.config/Audacity.

Written by the indexing model from the issue text.