fix(file-history-dialog): prevent long paths from displacing header controls

Constrain the dialog header/body layout so very long file paths don't push
or wrap the dialog controls. Results: the title is truncated with an
ellipsis but the full name is available on hover.

- CSS: add grid-template-columns and min-width:0 to dialog/body; force
  unified-dialog-header to avoid wrapping and set unified-dialog-text
  flex-basis:0; truncate h2 with overflow/ellipsis.
- Svelte: add a title attribute to the h2 so the full filename is shown
  on hover.

src/lib/components/FileHistoryDialog.svelte

@@ -52,7 +52,7 @@
       <div class="file-history-dialog-icon unified-dialog-icon" aria-hidden="true"><History size={19} /></div>
       <div class="file-history-dialog-heading unified-dialog-text">
         <span class="eyebrow">File history</span>
-        <h2 id="file-history-dialog-title">{fileName(filePath)}</h2>
+        <h2 id="file-history-dialog-title" title={fileName(filePath)}>{fileName(filePath)}</h2>
         <span class="file-history-dialog-path" title={filePath}>{filePath}</span>
       </div>
       <button class="dialog-icon-button" type="button" onclick={onClose} disabled={isBusy} aria-label="Close file history">