Fix intermittent TypeError during route transitions

Svelte 5's signal system can evaluate derived signals in child
components before parent {#if} blocks destroy them. When the
route store changes (e.g. from repo.commit to repo.patch), the
spread props update before the old component is unmounted, causing
commit to become undefined and Changeset to throw accessing
undefined.files.

Use optional chaining on Commit prop expressions so getters
return undefined instead of throwing, and guard Changeset
template with {#if diff} plus optional chaining on files
lookups.

src/views/repos/Changeset.svelte

@@ -7,11 +7,11 @@
   import Icon from "@app/components/Icon.svelte";
   import Observer, { intersection } from "@app/components/Observer.svelte";
 
-  export let diff: Diff;
-  export let files: Record<string, CommitBlob>;
+  export let diff: Diff | undefined = undefined;
+  export let files: Record<string, CommitBlob> | undefined = undefined;
   export let baseUrl: BaseUrl;
   export let repoId: string;
-  export let revision: string;
+  export let revision: string | undefined = undefined;
 
   let expanded = true;
 
@@ -73,63 +73,67 @@
   }
 </style>
 
-<div class="header">
-  <div class="summary">
-    <span>{diffDescription(diff.files)}</span>
-    with
-    <span class:additions={diff.stats.insertions > 0}>
-      {diff.stats.insertions}
-      {pluralize("insertion", diff.stats.insertions)}
-    </span>
-    and
-    <span class:deletions={diff.stats.deletions > 0}>
-      {diff.stats.deletions}
-      {pluralize("deletion", diff.stats.deletions)}
-    </span>
-  </div>
-  {#if diff.stats.filesChanged > 1}
-    <IconButton on:click={() => (expanded = !expanded)}>
-      {#if expanded === true}
-        <Icon name="collapse-vertical" />
-        <span class="global-hide-on-mobile-down">Collapse all</span>
-      {:else}
-        <Icon name="expand-vertical" />
-        <span class="global-hide-on-mobile-down">Expand all</span>
-      {/if}
-    </IconButton>
-  {/if}
-</div>
-
-<div class="diff-list">
-  <Observer let:filesVisibility let:observer>
-    {#each diff.files as file}
-      {@const path = "path" in file ? file.path : file.newPath}
-      <div class="file" use:intersection={observer} id={"observer:" + path}>
-        {#if "diff" in file}
-          <FileDiff
-            {repoId}
-            {baseUrl}
-            {revision}
-            {expanded}
-            filePath={path}
-            oldFilePath={"oldPath" in file ? file.oldPath : undefined}
-            fileDiff={file.diff}
-            headerBadgeCaption={file.status}
-            content={"new" in file ? files[file.new.oid]?.content : undefined}
-            oldContent={"old" in file
-              ? files[file.old.oid]?.content
-              : undefined}
-            visible={filesVisibility.has(path)} />
+{#if diff}
+  <div class="header">
+    <div class="summary">
+      <span>{diffDescription(diff.files)}</span>
+      with
+      <span class:additions={diff.stats.insertions > 0}>
+        {diff.stats.insertions}
+        {pluralize("insertion", diff.stats.insertions)}
+      </span>
+      and
+      <span class:deletions={diff.stats.deletions > 0}>
+        {diff.stats.deletions}
+        {pluralize("deletion", diff.stats.deletions)}
+      </span>
+    </div>
+    {#if diff.stats.filesChanged > 1}
+      <IconButton on:click={() => (expanded = !expanded)}>
+        {#if expanded === true}
+          <Icon name="collapse-vertical" />
+          <span class="global-hide-on-mobile-down">Collapse all</span>
         {:else}
-          <FileLocationChange
-            headerBadgeCaption={file.status}
-            oldPath={file.oldPath}
-            newPath={file.newPath}
-            {repoId}
-            {baseUrl}
-            {revision} />
+          <Icon name="expand-vertical" />
+          <span class="global-hide-on-mobile-down">Expand all</span>
         {/if}
-      </div>
-    {/each}
-  </Observer>
-</div>
+      </IconButton>
+    {/if}
+  </div>
+
+  <div class="diff-list">
+    <Observer let:filesVisibility let:observer>
+      {#each diff.files as file}
+        {@const path = "path" in file ? file.path : file.newPath}
+        <div class="file" use:intersection={observer} id={"observer:" + path}>
+          {#if "diff" in file}
+            <FileDiff
+              {repoId}
+              {baseUrl}
+              {revision}
+              {expanded}
+              filePath={path}
+              oldFilePath={"oldPath" in file ? file.oldPath : undefined}
+              fileDiff={file.diff}
+              headerBadgeCaption={file.status}
+              content={"new" in file
+                ? files?.[file.new.oid]?.content
+                : undefined}
+              oldContent={"old" in file
+                ? files?.[file.old.oid]?.content
+                : undefined}
+              visible={filesVisibility.has(path)} />
+          {:else}
+            <FileLocationChange
+              headerBadgeCaption={file.status}
+              oldPath={file.oldPath}
+              newPath={file.newPath}
+              {repoId}
+              {baseUrl}
+              {revision} />
+          {/if}
+        </div>
+      {/each}
+    </Observer>
+  </div>
+{/if}

src/views/repos/Commit.svelte

@@ -146,8 +146,8 @@
     <Changeset
       {baseUrl}
       repoId={repo.rid}
-      files={commit.files}
-      diff={commit.diff}
-      revision={commit.commit.id} />
+      files={commit?.files}
+      diff={commit?.diff}
+      revision={commit?.commit?.id} />
   </div>
 </Layout>