feat(core,gui): keep pages up while they reload

Pages no longer swap their content for a "Loading…" status on every load.
A new Loader keeps the last rows on screen and only shows the loading
state if the reply takes longer than 300 ms; a newer load always
supersedes an older one, so a late reply is dropped instead of painted.
Reloading what is on screen keeps it usable; loading something else greys
the old rows out and clears the selection, since their paths would
resolve against the new folder.

My files, search results and Trash now swap only the rows that changed,
so selection, scroll position and thumbnails survive a refresh. Activity
is one sectioned ListView: open conflicts first, then a section per day,
with the filter bar pinned above the list.

Polls give up after 5 s instead of 120 s. A daemon that is running but
doesn't answer now reads as "Not responding" in the app and the tray
rather than as disconnected.

crates/pdfs-gui/resources/style.css

@@ -93,6 +93,13 @@ scale.browser-status-meter trough { min-width: 104px; min-height: 6px; }
 /* Shared with Me: what a share lets me do, as a tinted pill beside the name. */
 .role-pill { padding: 2px 10px; border-radius: 999px; font-weight: 600; color: var(--accent-color); background-color: color-mix(in srgb, var(--accent-bg-color) 16%, transparent); }
 
+/* Activity: the feed sits on the window background as one card per row, under
+ * day headings, like a boxed list with its rows set apart. */
+listview.activity-feed { background: none; color: inherit; padding: 0 12px 18px 12px; }
+listview.activity-feed > row { padding: 0; margin-bottom: 6px; background: none; }
+listview.activity-feed > header { padding: 18px 6px 6px 6px; background: none; }
+listview.activity-feed row.card { border-radius: 12px; }
+
 /* Photos: how many timeline filters are on, as a small badge on the Filters button. */
 .filter-count { min-width: 18px; padding: 0 5px; border-radius: 999px; font-size: 0.8em; font-weight: 700; color: var(--accent-fg-color); background-color: var(--accent-bg-color); }
 /* The Places map: sea and land come in the map's own colours; markers (drawn