unable to access steamwork api

**Game information**

https://store.steampowered.com/app/646570/Slay_the_Spire/

**Distribution name and version where applicable**

fedora kinoite 40

**Flatpak info**
```
Flatpak 1.15.10
nvidia-560-35-03
default
host
```

**Problem description**

copied from https://github.com/ValveSoftware/steam-for-linux/issues/11333:

#### Your system information

* Steam client version (build number or date): 1726604483, Wed, Sep 18 5:03 AM UTC +09:00
* Distribution (e.g. Ubuntu): Fedora Kinoite 40
* Opted into Steam client beta?: No
* Have you checked for system updates?: Yes
* Steam Logs: [steam-logs.tar.gz](https://github.com/user-attachments/files/17309615/steam-logs.tar.gz)
* GPU: Nvidia
* steam installed via [flatpak](https://flathub.org/apps/com.valvesoftware.Steam)

#### Please describe your issue in as much detail as possible:

steamworks API fails to connect to running (flatpak) steam client.

#### Steps for reproducing this issue:

1. install [flatpak steam](https://flathub.org/apps/com.valvesoftware.Steam).
2. ran `ln -s ~/.var/app/com.valvesoftware.Steam/.steam ~/.steam` because AFAIK steam API uses `~/.steam`
3. attempted to run [StS mod uploader](https://steamcommunity.com/sharedfiles/filedetails/?id=1767940979): `java -jar mod-uploader.jar upload -w MarisaContinued/`
4. failed with:
```
[S_API FAIL] SteamAPI_Init() failed; SteamAPI_IsSteamRunning() failed.
[S_API FAIL] SteamAPI_Init() failed; unable to locate a running instance of Steam, or a local steamclient.so.
10월 09, 2024 11:13:18 오후 com.megacrit.moduploader.Uploader connectToSteam
심각: Steam API failed to initialize correctly.
```
5. if `mod-uploader.jar` is unavailable: the same error can be reproduced with https://github.com/ceifa/steamworks.js/ with [node.js](https://nodejs.org/en)
```sh
$ mkdir reproduce && cd reproduce
$ npm init es6 -y
$ npm install steamworks.js
$ echo 'import steamworks from "steamworks.js"

const client = steamworks.init(646570)
console.log(client.localplayer.getName())' > main.js
$ node main.js
[S_API] SteamAPI_Init(): SteamAPI_IsSteamRunning() did not locate a running instance of Steam.
[S_API] SteamAPI_Init(): Loaded '/var/home/scarf/.var/app/com.valvesoftware.Steam/.local/share/Steam/linux64/steamclient.so' OK.
IPC function call IClientUtils::SetAppIDForCurrentPipe took too long: 74 msec
Setting breakpad minidump AppID = 646570
SteamInternal_SetMinidumpSteamID: Caching Steam ID: 76561198168686419 [API loaded no]
목도리
```
6. retried step 4 inside flatpak (`flatpak run --command=sh com.valvesoftware.Steam`) but the result was same.

**Does this issue reproduce with native Steam**
No

Start by reproducing the failure with the provided `mod-uploader.jar` command and the `steamworks.js` `main.js` example. Compare the Flatpak Steam paths, including `~/.var/app/com.valvesoftware.Steam/.local/share/Steam/linux64/steamclient.so` and the `~/.steam` symlink, and check the result inside the Steam Flatpak shell. Done means identifying whether the Flatpak client can expose the Steamworks API or documenting the remaining limitation.

Written by the indexing model from the issue text.