RTL languages (Hebrew, Arabic) are unusable in the desktop client: no bidi handling in message bodies or composer

RTL text is currently unusable in the desktop client. Hebrew and Arabic render with punctuation, brackets and numbers on the wrong edge of every line.

### Steps to reproduce

1. Send a message containing Hebrew or Arabic text ending in a period, parentheses or quotes.
2. Observe the trailing punctuation renders at the wrong edge of the line.
3. Same in the composer while typing.

**Environment:** Buzz 0.5.3, macOS (Apple Silicon). Also reproduced in a dev build at `28ae6cd`.

### Current


The sentence-final period sits at the *start* of line 1, the opening quote is on the wrong side, and the Arabic closing paren has jumped to the left edge.

### With the fix below


The English line is unchanged.

### Root cause

There is no bidi handling anywhere in the desktop client:

- [`desktop/index.html#L2`](https://github.com/block/buzz/blob/28ae6cd2174309529305724e455c7ca082f6fe4b/desktop/index.html#L2) is `` with no `dir`
- `dir="auto"` appears 0 times in the repository
- [`markdown.css`](https://github.com/block/buzz/blob/28ae6cd2174309529305724e455c7ca082f6fe4b/desktop/src/shared/styles/globals/markdown.css) sets no direction on any element
- [`markdown.tsx#L1615`](https://github.com/block/buzz/blob/28ae6cd2174309529305724e455c7ca082f6fe4b/desktop/src/shared/ui/markdown.tsx#L1615) renders a bare `

`

Every block therefore resolves at bidi paragraph level 0 (LTR), so neutral characters at a run boundary take the paragraph direction instead of the surrounding text's.

### Proposed fix

One rule in `markdown.css`, scoped to `.message-markdown`:

```css
.message-markdown p,
.message-markdown li,
.message-markdown blockquote,
.message-markdown h1,
.message-markdown h2,
.message-markdown h3,
.message-markdown h4,
.message-markdown h5,
.message-markdown h6,
.message-markdown td,
.message-markdown th {
unicode-bidi: plaintext;
}
```

`unicode-bidi: plaintext` resolves direction per block from the first strong character, which is the correct behaviour for a client carrying mixed LTR/RTL content. LTR-only messages are unaffected. Code blocks are deliberately excluded, so `pre` and `code` stay LTR.

**This fixes reading and typing with the same rule.** `MESSAGE_MARKDOWN_CLASS` is applied both to the rendered message wrapper ([`markdown.tsx#L1945`](https://github.com/block/buzz/blob/28ae6cd2174309529305724e455c7ca082f6fe4b/desktop/src/shared/ui/markdown.tsx#L1945)) and to the TipTap editor root ([`useRichTextEditor.ts#L492`](https://github.com/block/buzz/blob/28ae6cd2174309529305724e455c7ca082f6fe4b/desktop/src/features/messages/lib/useRichTextEditor.ts#L492)).

### Verification

Screenshots above are from the actual desktop app via `just desktop-screenshot` at `28ae6cd`, with and without the rule, same message and same clip region. `just desktop-check` passes.

Happy to open a PR.