fix: workspace panel close button — no duplicate X on desktop, mobile X respects file preview (#414)

* fix: workspace panel close button — no duplicate X on desktop, mobile X respects file preview

Two bugs fixed in the workspace right panel:

1. Duplicate X on desktop (bug): #btnClearPreview (the X icon) was always
   visible alongside #btnCollapseWorkspacePanel (the chevron), producing two
   close controls at once. Fixed in syncWorkspacePanelUI() — on desktop, the X
   is now hidden when no file preview is open (display:none), and only shown
   when the user is viewing a file. The chevron remains as the sole close
   control in browse mode.

2. Mobile X collapses panel instead of dismissing file (bug): .mobile-close-btn
   was calling closeWorkspacePanel() directly, which collapsed the whole panel
   even when a file was open. Changed to handleWorkspaceClose(), which already
   has the correct two-step logic: clear preview first, close panel only if
   no preview is visible.

Files changed:
- static/boot.js: syncWorkspacePanelUI() hides btnClearPreview on desktop
  when hasPreview is false, guarded by !isCompact so mobile is unaffected
- static/index.html: mobile-close-btn onclick changed from
  closeWorkspacePanel() to handleWorkspaceClose()
- tests/test_sprint44.py: 10 new regression tests
- tests/test_mobile_layout.py: updated test_workspace_close_button_present()
  to accept handleWorkspaceClose() as the valid onclick target

* fix: widen test_server_delete_invalidates_index window to 1200 chars

The test extracted a 600-char window starting from the session/delete
handler to check for SESSION_INDEX_FILE. Commit 3cc5839 added session_id
character validation and path traversal guards before the unlink call,
pushing SESSION_INDEX_FILE to ~764 chars from the match — beyond the
600-char limit, causing the test to fail on CI.

Widened the window to 1200 chars, which accommodates any reasonable
amount of guard code before the SESSION_INDEX_FILE.unlink() call.

* docs: v0.50.33 release — version badge and CHANGELOG

---------

Co-authored-by: Nathan Esquenazi <nesquena@gmail.com>

static/boot.js

@@ -118,6 +118,10 @@ function syncWorkspacePanelUI(){
   if(clearBtn){
     clearBtn.disabled=!isOpen;
     clearBtn.title=hasPreview?'Close preview':'Hide workspace panel';
+    // On desktop, only show the X button when a file preview is open.
+    // In browse mode the chevron (btnCollapseWorkspacePanel) already serves
+    // as the close control, so showing both produces a duplicate X.
+    if(!isCompact) clearBtn.style.display=hasPreview?'':'none';
   }
 }