Problem

The left sidebar tab buttons (Scenes / Story) and several toolbar buttons have no
distinct disabled state. When a state actually blocks interaction (e.g. Story tab with
no content yet, or toolbar buttons during an export), the button still looks active,
which makes failures feel like bugs.

Observed

• Buttons rely on hover state for feedback; without hover they look identical whether
  they're enabled or disabled.
• There's no aria-disabled / disabled convention applied consistently.

Proposed

• Add a shared .btn--disabled style (reduced opacity, cursor: not-allowed, no hover
  background change).
• Audit every button in: TitleBar.svelte, LeftPanel.svelte, Editor.svelte status
  bar, and export modal action row.
• Apply disabled attribute + the style consistently.

Acceptance

• Every button across the app has a visibly distinct disabled state.
• Disabled buttons don't show a hover background.
• Screen readers announce disabled state.