Fix crash on startup when a session file cannot be read

[hemanpa]

ScintillaNext::fromFile() returns a null pointer when the file cannot be read from
disk, but SessionManager used the returned pointer without checking it.

If a session references a file that still exists but cannot be opened — for example
after macOS resets folder access permissions following an upgrade — the null pointer
is dereferenced in EditorManager::setupEditor() and the application crashes during
startup, before the main window is shown. It then crashes on every subsequent launch,
leaving no way to recover other than deleting the settings.

This guards the three fromFile() call sites so an unreadable file is skipped and a
warning logged, which is what EditorManager::createEditorFromFile() already does.

It also guards the two findChild<BookMarkDecorator*>() results, which can also
return null — that is the code path in the crash log attached to the issue.

Testing

macOS 26.6, Qt 6.11.2, Debug build. Created a session referencing a chmod 000 file:

• Before: EXC_BAD_ACCESS (code=1, address=0x0) at
  ScintillaEdit::clearCmdKey(this=0x0, keyDefinition=309)
• After: the application starts, logs
  could not be read from disk, ignoring this file for session loading,
  and skips the file

Also verified a readable session file still restores normally, including bookmarks
round-tripping correctly through save and load.

Fixes #845

[hemanpa]

Heads up on the two failing checks — I believe both are pre-existing on master
rather than caused by this change:

• Check for spelling errors: alot in a comment at src/dialogs/MainWindow.cpp:742,
  introduced in b4c6f51. This check has been failing on master for the last few
  commits. This PR only touches src/SessionManager.cpp. Opened Fix spelling error in zoom out comment #1124 to fix the typo
  separately.
• build (macos-latest, 6.5) and (macos-latest, 6.8): both also fail on master
  (macOS 6.10, all Ubuntu and all Windows jobs pass). Looks related to what Improve MacOS build #1119 is
  addressing.

Everything that currently passes on master also passes here.