Add a whiteboard file picker and force-refresh

File picker: an optional WebDAV browse root in Settings
(User.webdav_base_url) plus a plain-PROPFIND directory listing
(webdav_client.list_directory) power a "Browse..." panel on a frame's
Whiteboard tab, so a file can be clicked into rather than typing its
exact WebDAV URL. Manual URL entry still works unchanged either way.

Force-refresh: get_or_refresh_whiteboard takes a force flag that skips
the fetch throttle entirely; the Whiteboard tab's refresh button now
passes it, so clicking it always re-fetches and re-renders instead of
possibly just re-showing the same cached image from within the last
~20 minutes.

server/app/webdav_client.py

@@ -14,11 +14,21 @@ philosophy as calendar_feed.py/caldav_client.py.
 
 from __future__ import annotations
 
+from urllib.parse import unquote, urljoin, urlsplit, urlunsplit
+from xml.etree import ElementTree
+
 import httpx
 
 HTTP_TIMEOUT_S = 15.0
 FETCH_MAX_BYTES = 10 * 1024 * 1024  # a whiteboard scene is KB, not MB -- sanity cap, not an expected size
 
+_DAV_NS = "DAV:"
+_PROPFIND_BODY = (
+    '<?xml version="1.0" encoding="utf-8" ?>'
+    f'<d:propfind xmlns:d="{_DAV_NS}"><d:prop>'
+    "<d:displayname/><d:resourcetype/></d:prop></d:propfind>"
+)
+
 
 class WebDavError(Exception):
     """Fetch failed -- network, auth, a missing file, or an oversized
@@ -44,3 +54,69 @@ def fetch_file(url: str, username: str, password: str) -> bytes:
             return b"".join(chunks)
     except httpx.HTTPError as e:
         raise WebDavError(str(e)) from e
+
+
+def list_directory(url: str, username: str, password: str) -> list[dict]:
+    """One level of a WebDAV directory listing -- name/url/is_dir for
+    each entry, folders first then alphabetical. Powers the whiteboard
+    file-picker (browse instead of paste the exact file URL); a plain
+    Depth-1 PROPFIND for displayname + resourcetype is all that needs,
+    same "just enough protocol, not a full WebDAV client" scope as
+    fetch_file above."""
+    try:
+        resp = httpx.request(
+            "PROPFIND", url, auth=(username, password), timeout=HTTP_TIMEOUT_S,
+            follow_redirects=True, headers={"Depth": "1", "Content-Type": "application/xml"},
+            content=_PROPFIND_BODY,
+        )
+        resp.raise_for_status()
+    except httpx.HTTPError as e:
+        raise WebDavError(str(e)) from e
+
+    try:
+        root = ElementTree.fromstring(resp.content)
+    except ElementTree.ParseError as e:
+        raise WebDavError(f"Server returned an unparseable directory listing: {e}") from e
+
+    self_path = urlsplit(url).path.rstrip("/")
+    entries = []
+    for response_el in root.findall(f"{{{_DAV_NS}}}response"):
+        href_el = response_el.find(f"{{{_DAV_NS}}}href")
+        if href_el is None or not href_el.text:
+            continue
+        href = href_el.text
+        if urlsplit(href).path.rstrip("/") == self_path:
+            continue  # the listed directory's own entry, not a child
+
+        propstat = response_el.find(f"{{{_DAV_NS}}}propstat")
+        prop = propstat.find(f"{{{_DAV_NS}}}prop") if propstat is not None else None
+        resourcetype = prop.find(f"{{{_DAV_NS}}}resourcetype") if prop is not None else None
+        is_dir = resourcetype is not None and resourcetype.find(f"{{{_DAV_NS}}}collection") is not None
+
+        displayname_el = prop.find(f"{{{_DAV_NS}}}displayname") if prop is not None else None
+        name = (displayname_el.text or "").strip() if displayname_el is not None else ""
+        if not name:
+            name = unquote(href.rstrip("/").rsplit("/", 1)[-1])
+        if not name:
+            continue
+
+        entries.append({"name": name, "url": urljoin(url, href), "is_dir": is_dir})
+
+    entries.sort(key=lambda e: (not e["is_dir"], e["name"].casefold()))
+    return entries
+
+
+def parent_directory_url(base_url: str, current_url: str) -> str | None:
+    """One level up from `current_url`, or None if `current_url` is
+    already at (or above) `base_url` -- the file-picker's "Up" button
+    doesn't wander outside the folder the user configured as their
+    browse root in Settings."""
+    base_path = urlsplit(base_url).path.rstrip("/")
+    current = urlsplit(current_url)
+    current_path = current.path.rstrip("/")
+    if len(current_path) <= len(base_path):
+        return None
+    parent_path = current_path.rsplit("/", 1)[0] or "/"
+    if len(parent_path) < len(base_path):
+        parent_path = base_path
+    return urlunsplit((current.scheme, current.netloc, parent_path + "/", "", ""))