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/routers/api_frames.py

@@ -25,7 +25,7 @@ from pydantic import BaseModel
 from sqlalchemy import select
 from sqlalchemy.orm import Session
 
-from .. import calendar_render, gitea_releases, photo_queue, quiet_hours, weather
+from .. import calendar_render, gitea_releases, photo_queue, quiet_hours, weather, webdav_client
 from ..auth import require_frame_control, require_frame_view, require_user_api
 from ..db import frame_locked, get_db
 from ..image_pipeline import (
@@ -52,6 +52,7 @@ from .common import (
     list_assets,
     require_configured,
     valid_http_url,
+    webdav_creds_for,
 )
 
 logger = logging.getLogger(__name__)
@@ -650,15 +651,51 @@ def api_whiteboard_source(
     return {"status": "saved", "url": body.url}
 
 
+@router.get("/api/frames/{frame_id}/whiteboard-browse")
+def api_whiteboard_browse(
+    request: Request,
+    url: str | None = None,
+    frame: Frame = Depends(require_frame_view),
+    db: Session = Depends(get_db),
+):
+    """One level of a WebDAV directory listing, using the calling user's
+    own credentials (never the frame's saved whiteboard_user_id -- this
+    is "help me find a file in MY account", same person as whoever would
+    go on to Save it, before that's even happened) -- powers the file
+    picker on the Whiteboard tab as an alternative to pasting a URL.
+    `url` omitted/None starts from the user's webdav_base_url (see
+    models.py's User docstring); passing back a previous response's
+    `entries[].url` (for a folder) descends into it."""
+    user = require_user_api(request, db)
+    creds = webdav_creds_for(user)
+    if creds is None:
+        raise HTTPException(400, "Set up WebDAV credentials in Settings first")
+    target = url or user.webdav_base_url
+    if not target:
+        raise HTTPException(400, "Set a WebDAV browse root in Settings first, or paste the file URL directly")
+    if not valid_http_url(target):
+        raise HTTPException(400, "Browse URL must be a plain http:// or https:// URL")
+    try:
+        entries = webdav_client.list_directory(target, creds[0], creds[1])
+    except webdav_client.WebDavError as e:
+        raise HTTPException(502, f"Could not browse: {e}")
+    base = user.webdav_base_url or target
+    parent_url = webdav_client.parent_directory_url(base, target)
+    return {"current_url": target, "parent_url": parent_url, "entries": entries}
+
+
 @router.get("/api/frames/{frame_id}/preview/whiteboard")
-def api_preview_whiteboard(frame: Frame = Depends(require_frame_view), db: Session = Depends(get_db)):
+def api_preview_whiteboard(
+    force: bool = False, frame: Frame = Depends(require_frame_view), db: Session = Depends(get_db)
+):
     """The same throttled fetch/render cache a live device request would
     use, run through the same panel composition/quantization pipeline
     (see routers/device.py's _render_whiteboard_mode) -- "how it will
     look on the frame" (dithered, letterboxed), not just the raw
     Excalidraw export, same convention as preview/rendered and
-    preview/calendar."""
-    png_bytes = get_or_refresh_whiteboard(db, frame)
+    preview/calendar. force=True (the "Refresh now" button, as opposed
+    to just reopening this tab) bypasses the fetch throttle."""
+    png_bytes = get_or_refresh_whiteboard(db, frame, force=force)
     if png_bytes is None:
         if not frame.whiteboard_url:
             raise HTTPException(400, "No whiteboard configured on this frame yet")