feat(history): make branch filter groups collapsible

Add collapsible local and remote branch groups to the branch
visibility dialog, including toggle buttons and selection counts.
Introduce comprehensive dialog styling and responsive rules to match
the app UI. Opening the dialog now defaults to local open and remote
closed for faster access.

- Add CSS for branch-filter dialog layout, theming, and responsiveness
- Implement group toggles with chevrons and visible selected counts
- Default to local group open and remote group closed on dialog open

src/lib/components/HistoryPanel.svelte

@@ -117,6 +117,8 @@
   let customVisibleBranches = $state<Set<string>>(new Set());
   let loadedVisibilityRepository = $state("");
   let branchDialogOpen = $state(false);
+  let localBranchGroupOpen = $state(true);
+  let remoteBranchGroupOpen = $state(false);
   let expandedRefsCommitHash = $state("");
   let panelElement = $state<HTMLElement | null>(null);
   let contextCommit = $state<GitCommit | null>(null);
@@ -428,6 +430,8 @@
   }
 
   function openBranchDialog() {
+    localBranchGroupOpen = true;
+    remoteBranchGroupOpen = false;
     branchDialogOpen = true;
   }
 
@@ -1202,32 +1206,62 @@
 
       <div class="branch-filter-dialog-list">
         {#if localBranchNames.length > 0}
-          <span class="branch-filter-group-label">Local</span>
-          {#each localBranchNames as branch}
-            <label class:muted={!branchIsVisible(branch)} class="branch-filter-option" title={branch}>
-              <input
-                type="checkbox"
-                checked={branchIsVisible(branch)}
-                onchange={() => toggleGraphBranch(branch)}
-              />
-              <GitBranch size={14} aria-hidden="true" />
-              <span>{branch}</span>
-            </label>
-          {/each}
+          <section class="branch-filter-group">
+            <button
+              class="branch-filter-group-toggle"
+              type="button"
+              aria-expanded={localBranchGroupOpen}
+              onclick={() => { localBranchGroupOpen = !localBranchGroupOpen; }}
+            >
+              {#if localBranchGroupOpen}<ChevronDown size={14} aria-hidden="true" />{:else}<ChevronRight size={14} aria-hidden="true" />{/if}
+              <span>Local</span>
+              <em>{localBranchNames.filter(branchIsVisible).length}/{localBranchNames.length}</em>
+            </button>
+            {#if localBranchGroupOpen}
+              <div class="branch-filter-group-list">
+                {#each localBranchNames as branch}
+                  <label class:muted={!branchIsVisible(branch)} class="branch-filter-option" title={branch}>
+                    <input
+                      type="checkbox"
+                      checked={branchIsVisible(branch)}
+                      onchange={() => toggleGraphBranch(branch)}
+                    />
+                    <GitBranch size={14} aria-hidden="true" />
+                    <span>{branch}</span>
+                  </label>
+                {/each}
+              </div>
+            {/if}
+          </section>
         {/if}
         {#if remoteBranchNames.length > 0}
-          <span class="branch-filter-group-label">Remote</span>
-          {#each remoteBranchNames as branch}
-            <label class:muted={!branchIsVisible(branch)} class="branch-filter-option remote" title={branch}>
-              <input
-                type="checkbox"
-                checked={branchIsVisible(branch)}
-                onchange={() => toggleGraphBranch(branch)}
-              />
-              <GitBranch size={14} aria-hidden="true" />
-              <span>{branch}</span>
-            </label>
-          {/each}
+          <section class="branch-filter-group">
+            <button
+              class="branch-filter-group-toggle"
+              type="button"
+              aria-expanded={remoteBranchGroupOpen}
+              onclick={() => { remoteBranchGroupOpen = !remoteBranchGroupOpen; }}
+            >
+              {#if remoteBranchGroupOpen}<ChevronDown size={14} aria-hidden="true" />{:else}<ChevronRight size={14} aria-hidden="true" />{/if}
+              <span>Remote</span>
+              <em>{remoteBranchNames.filter(branchIsVisible).length}/{remoteBranchNames.length}</em>
+            </button>
+            {#if remoteBranchGroupOpen}
+              <div class="branch-filter-group-list">
+                {#each remoteBranchNames as branch}
+                  <label class:muted={!branchIsVisible(branch)} class="branch-filter-option" title={branch}>
+                    <input
+                      type="checkbox"
+                      checked={branchIsVisible(branch)}
+                      onchange={() => toggleGraphBranch(branch)}
+                    />
+                    <GitBranch size={14} aria-hidden="true" />
+                    <span>{branch}</span>
+                  </label>
+                {/each}
+              </div>
+            {/if}
+          </section>
         {/if}
       </div>
     </div>