Fix header spacing in issue-write and pr-write UIs

Add proper spacing between icon, title text, and repo name in the
header bar for both issue-write and create-pull-request forms.

Author: mattdholloway

ui/src/apps/issue-write/App.tsx

@@ -241,11 +241,13 @@ function CreateIssueApp() {
         borderBottomStyle="solid"
         borderBottomColor="border.default"
       >
-        <IssueOpenedIcon size={16} />
-        <Text sx={{ fontWeight: "semibold" }}>
+        <Box sx={{ color: "fg.default", flexShrink: 0, display: "flex" }}>
+          <IssueOpenedIcon size={16} />
+        </Box>
+        <Text sx={{ fontWeight: "semibold", whiteSpace: "nowrap" }}>
           {isUpdateMode ? `Update issue #${issueNumber}` : "New issue"}
         </Text>
-        <Text sx={{ color: "fg.muted", fontSize: 0 }}>
+        <Text sx={{ color: "fg.muted", fontSize: 0, ml: 1 }}>
           {owner}/{repo}
         </Text>
       </Box>

ui/src/apps/pr-write/App.tsx

@@ -231,9 +231,11 @@ function CreatePRApp() {
           borderBottomStyle="solid"
           borderBottomColor="border.default"
         >
-          <GitPullRequestIcon size={16} />
-          <Text sx={{ fontWeight: "semibold" }}>New pull request</Text>
-          <Text sx={{ color: "fg.muted", fontSize: 0 }}>
+          <Box sx={{ color: "fg.default", flexShrink: 0, display: "flex" }}>
+            <GitPullRequestIcon size={16} />
+          </Box>
+          <Text sx={{ fontWeight: "semibold", whiteSpace: "nowrap" }}>New pull request</Text>
+          <Text sx={{ color: "fg.muted", fontSize: 0, ml: 1 }}>
             {owner}/{repo}
           </Text>
           {head && base && (