fix(paths): one home-prefix helper, so labels abbreviate on both platforms

The rule "show ~/project rather than /home/<user>/project" had three
implementations in the frontend, two of them platform-specific in opposite
directions, so each looked correct to whoever wrote it.

- The Run menu's Recent Sessions rows matched /home/<user>/ only. On macOS
  nothing was stripped, so every row spent its first ~19 characters on an
  identical /Users/<user>/ prefix and the left-to-right ellipsis removed the
  tail that identifies the row. That is #273, reported by @jordan8037310, who
  also traced why the menu's 250px cap made it worse: the width was chosen on
  the assumption the abbreviation had run.
- The case-manage list matched /Users/<user> only, the mirror image, so on a
  Linux host no case path was ever abbreviated there. Unreported.

Both now call _shortenHomePath(), which was already correct for both layouts
and already used by the Resume list, Cmd+K, the desktop home rail and the phone
overview. Its regex collapses to one alternation with a lookahead, so a path
that is exactly $HOME renders "~" instead of being left raw, matching what the
case-manage list used to do on macOS.

test/home-path-abbreviation.test.ts pins the helper on both layouts and the
rendered case-manage label, and fails if a fourth copy of the pattern appears in
src/web/public. The Run-menu guard counts helper calls rather than pinning a
source line, so it survives the row restructure in #274.

test/run-mode-ui.test.ts gains a _shortenHomePath stub: its harness loads
session-ui.js without terminal-ui.js, which the real app never does.

Verified against an isolated instance with 27 real cases and 50 history rows:
27 of 27 case paths and 17 of 20 Run menu rows abbreviate, the other 3 are
/tmp paths that correctly stay raw, tooltips keep the full path, no page errors.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

test/run-mode-ui.test.ts

@@ -760,6 +760,11 @@ describe('case selector refresh', () => {
     ];
     app.showToast = vi.fn();
 
+    // deleteCase re-renders the case-manage list, whose path label goes through
+    // _shortenHomePath. That method lives in terminal-ui.js, which this harness
+    // does not load (the real app always has it: load order 7 before 12).
+    app._shortenHomePath = (p: string) => p;
+
     await app.deleteCase('deleted-case');
 
     expect(quickStartCase.blur).toHaveBeenCalled();