Bugfix: hide Brief cursor while column widths load

Entering a new dir cleared `columnWidths`, so for one frame every column rendered at the `capPx` fallback (≈ full pane width), making the cursor stripe momentarily fill the entire pane.

- Gate `is-under-cursor` on `columnWidths.length > 0` so no cursor stripe renders at the fallback width.
- Skip the 50 ms coalesce on the first fetch (empty `columnWidths`) so the cursor-hidden gap is as short as possible; subsequent re-fetches still coalesce.

Author: vdavid

apps/desktop/src/lib/file-explorer/views/BriefList.svelte

@@ -358,6 +358,13 @@
     function fetchColumnWidths() {
         if (!listingId || itemsPerColumn <= 0) return
         cancelPendingFetch()
+        // First fetch (no widths yet, for example after entering a new dir) fires
+        // immediately so the cursor-hidden gap is as short as possible. Subsequent
+        // re-fetches keep the 50 ms coalesce to absorb resize bursts.
+        if (columnWidths.length === 0) {
+            void doFetchColumnWidths(false)
+            return
+        }
         pendingFetchTimer = setTimeout(() => {
             pendingFetchTimer = null
             void doFetchColumnWidths(false)
@@ -787,7 +794,7 @@
                             <div
                                 id={`file-${String(globalIndex)}`}
                                 class="file-entry"
-                                class:is-under-cursor={globalIndex === cursorIndex}
+                                class:is-under-cursor={globalIndex === cursorIndex && columnWidths.length > 0}
                                 class:is-selected={selectedIndices.has(globalIndex)}
                                 class:is-striped={stripedRows && globalIndex % 2 === 1}
                                 class:is-restricted={fileIsRestricted}

apps/desktop/src/lib/file-explorer/views/CLAUDE.md

@@ -93,7 +93,10 @@ array (`$derived`) drives all virtual-scroll math: `totalSize` is the final pref
 binary-searches `prefixSums` for the visible range, and `getScrollToPositionVariable` looks up exact column edges.
 Scrollbar size and cursor visibility now agree with what's actually rendered. `transition: width 300ms ease` still
 animates width changes within a directory; nav resets snap via the `skipTransition` 2-rAF trick. While widths are in
-flight (first paint, after `FontMetricsNotReady`), every column renders at `capPx` as a fallback.
+flight (first paint, after `FontMetricsNotReady`), every column renders at `capPx` as a fallback, and the cursor
+highlight is suppressed until `columnWidths.length > 0` so the user doesn't see a full-pane-wide cursor stripe for a
+frame. The initial fetch after a dir change skips the 50 ms coalesce so that gap is as short as possible; re-fetches
+during resize keep the coalesce.
 
 **Decision**: A single `$effect` keeps the cursor in view, depending on `cursorIndex`, `containerWidth`,
 `containerHeight`, and `columnWidths.length` **Why**: With exact prefix-sum math, every input that could move the