feat(chat): add quick-open file icon in tool card header

Add a small external-link icon next to the tool display name in the
collapsed tool card header (Write/Edit/MultiEdit/ApplyPatch), so the
target file can be opened in the side panel (web/desktop) or editor
(VS Code) without expanding the card.

On web/desktop (no runtime.editor), the icon falls back to
useUIStore.openContextFile{AtLine} + mobileActions.openFiles(), matching
the existing openEntryFile pattern. The existing handleMainClick only
opens the file when runtime.editor is available, so this icon is the
first way to open a file from the tool header in the browser.

Path resolution reuses getPrimaryToolPath + toAbsoluteFilePath; the diff
tools also resolve the first changed line and primary diff via the
existing getFirstChangedLineFromMetadata / getPrimaryDiffFromMetadata
helpers. The icon stops click propagation so the card-toggle-on-click
behavior is preserved.

Adds the chat.toolPart.openFile i18n key across all 11 locales.

packages/ui/src/lib/i18n/messages/uk.ts

@@ -2300,6 +2300,7 @@ export const dict: Record<I18nKey, string> = {
   "chat.toolPart.showRawJson": "Показати сирий JSON",
   "chat.toolPart.showFormattedJson": "Показати форматований JSON",
   "chat.toolPart.showNavigableJson": "Показати навігаційний JSON",
+  "chat.toolPart.openFile": "Відкрити файл",
   "chat.toolPart.openFileAtFirstChange": "Відкрити файл на першій зміні",
   "chat.toolPart.openFileDiff": "Відкрити diff файлу",
   "chat.toolPart.copyOutput": "Скопіювати вивід",