[Feature]: Dark mode for PDF previews

[edemaine]

Problem or use case

Bright PDF pages can cause discomfort for people, especially those with light sensitivity, and especially when reading alongside a conversation in T3 Code’s dark theme. Dark mode would make longer reading sessions more comfortable.

Proposed behavior

Add a PDF appearance setting with three choices:

• Match app (default): use dark PDF rendering when T3 Code is dark; show original document colors when T3 Code is light.
• Original colors: always preserve the document’s appearance.
• Dark: always display light text on a dark background.

In each PDF preview, allow switching between dark rendering and original colors for that document. This override lasts while the preview is open; newly opened previews use the default. It keeps original colors easily accessible for diagrams, images, and other content where color matters.

There are a few ways to do color inversion. We can probably just choose one. My favorite is this filter from Dark Reader's Filter+, but filter: invert(1) hue-rotate(180deg) is good and simple. Even filter: invert(1) isn't bad.

Smallest useful scope

Apply this wherever T3 Code previews PDFs. Preserve text selection, search, zoom, and reading position when changing appearance.