feat(FN-903): add middle truncation utility for file paths in dashboard

- Add truncatePath utility function with configurable segments and placeholder
- Apply middle truncation to file paths in ChangedFilesModal sidebar
- Apply middle truncation to file paths in TaskChangesTab
- Remove unused CSS truncation styles replaced by the new utility
- Add comprehensive test suite for truncatePath (106 tests)

packages/dashboard/app/styles.css

@@ -3560,7 +3560,6 @@ body {
   flex: 1;
   min-width: 0;
   overflow: hidden;
-  text-overflow: ellipsis;
   white-space: nowrap;
   font-family: var(--font-mono);
   font-size: 12px;
@@ -9438,7 +9437,6 @@ html .column.drag-over * {
   flex: 1;
   min-width: 0;
   overflow: hidden;
-  text-overflow: ellipsis;
   white-space: nowrap;
 }
 
@@ -9869,7 +9867,6 @@ html .column.drag-over * {
   flex: 1;
   min-width: 0;
   overflow: hidden;
-  text-overflow: ellipsis;
   white-space: nowrap;
 }
 
@@ -15101,7 +15098,6 @@ html .column.drag-over * {
   flex: 1;
   min-width: 0;
   overflow: hidden;
-  text-overflow: ellipsis;
   white-space: nowrap;
   font-family: ui-monospace, SFMono-Regular, "SF Mono", Menlo, Consolas, monospace;
 }