Feed date filter: early-stop global feed load, fix cross-date validation, show Loading while fetching

- Stop fetching combined-feed pages once the oldest activity in a batch predates
  the from-date (feed is newest-first, so everything needed is already loaded)
- Show "Loading…" instead of "No activities found" while eager-load is in progress
- Constrain From max to customTo (or today) and To min to customFrom so the
  range can't be inverted via the date pickers

site/src/components/ActivityFeed.svelte

@@ -161,6 +161,8 @@
   $: if ((query.trim() || customFrom || customTo || datePre !== 'all') && feedNextPage > 0 && !loadingAllFeedPages) {
     loadingAllFeedPages = true;
     (async () => {
+      // Snapshot at loop start — customFrom may change while we're fetching.
+      const fromFilter = customFrom;
       while (feedNextPage > 0) {
         const page = feedNextPage;
         feedNextPage = page < feedTotalPages ? page + 1 : 0;
@@ -171,6 +173,12 @@
           all = [...existing.values()].sort((a, b) =>
             (b.started_at ?? '').localeCompare(a.started_at ?? ''),
           );
+          // Feed is sorted newest-first. Once the oldest activity in this page
+          // predates our from-filter, everything needed is already loaded.
+          if (fromFilter && fresh.length > 0) {
+            const oldest = fresh.reduce((m, a) => (a.started_at ?? '') < (m.started_at ?? '') ? a : m);
+            if ((oldest.started_at ?? '') < fromFilter) { feedNextPage = 0; break; }
+          }
         } catch { /* ignore — partial results still useful */ }
       }
       loadingAllFeedPages = false;
@@ -260,7 +268,7 @@
       <input
         type="date"
         bind:value={customFrom}
-        max={today}
+        max={customTo || today}
         on:change={() => { datePre = 'all'; }}
         class="bg-transparent text-white text-sm focus:outline-none [color-scheme:dark]"
       />
@@ -270,6 +278,7 @@
       <input
         type="date"
         bind:value={customTo}
+        min={customFrom}
         max={today}
         on:change={() => { datePre = 'all'; }}
         class="bg-transparent text-white text-sm focus:outline-none [color-scheme:dark]"
@@ -332,7 +341,7 @@
   <p class="text-red-400 text-center py-12">Could not load activities: {error}</p>
 {:else if withSearch.length === 0}
   <p class="text-zinc-500 text-center py-12">
-    {#if query.trim()}No activities match "{query.trim()}".{:else}No activities found.{/if}
+    {#if loadingAllFeedPages || loadingAllShards}Loading…{:else if query.trim()}No activities match "{query.trim()}".{:else}No activities found.{/if}
   </p>
 {:else}
   <div class="grid grid-cols-1 sm:grid-cols-2 lg:grid-cols-3 gap-4">