fix(editor): allow saving from reading mode

[PathGao]

Summary

With a document that was never saved, Ctrl/Cmd+S in reading mode did nothing at all.

toggleEdit only runs its save-or-confirm flow for tabs that already have a path (if (tab.isDirty && tab.path !== '')), so an untitled buffer switches to reading mode with its content still unsaved. The keydown handler then suppressed the shortcut behind if (isEditing || isSplit) — the one action that could rescue that text was the one action the mode refused. Nothing on screen suggests the shortcut is unavailable; it just silently does nothing, and the only recovery is knowing to switch back to the editor first.

Saving is not mode-specific. The guard now asks whether there is anything to write rather than which pane happens to be visible:

• an untitled buffer saves from any mode — documentSession.saveContent already opens the Save dialog for a pathless tab, so reading mode needs no extra plumbing;
• a saved, unmodified document stays a no-op, so the shortcut cannot churn its mtime and wake the file watcher (which, in live mode, would reload the tab underneath the user);
• the HOME tab is excluded by the same condition without being special-cased — it carries a sentinel path and is never dirty.

preventDefault now runs unconditionally, including for the no-op case. Previously reading mode let the shortcut fall through to the webview's own Save Page dialog.

Credit

Reported by @dayeggpi in #168. That PR also proposed two other changes, both since overtaken by other work: the "autosave + confirm before save prompts every 1.5 s" loop is fixed on master (auto-save is now disabled outright while confirmBeforeSave is on), and the menu Save entry it edited no longer exists — the macOS File submenu was removed in a later refactor. This lifts the one finding that is still live.

Validation

• npm run check — 0 errors, 0 warnings
• npm test — 168/168, including a new scripts/saveFromReadingMode.test.ts covering the untitled case, the clean-document no-op, unconditional preventDefault, and HOME staying unsavable