renderer: apply font-thicken to IME preedit text (#14230)

Fixes #13758

### Problem

With `font-thicken = true` on macOS, IME preedit (composing) text
renders noticeably thinner than committed text.

### Cause & Fix
Committed and preedit text diverge in `src/renderer/generic.zig`:

```text
Committed: Shaper -> addGlyph -> renderGlyph(..., { .thicken = config.font_thicken, ... })
Preedit: State -> addPreeditCell -> renderCodepoint(..., { .grid_metrics })
```

Fix: Pass `self.config.font_thicken` and
`self.config.font_thicken_strength` in `addPreeditCell`.

### Testing

- Verified on macOS with `font-thicken = true`.
- Verified with `zig fmt --check`.

### AI Disclosure

AI (gemini-3.8-flash) assisted in tracing the call path; manually
verified and tested.

src/renderer/generic.zig

@@ -3576,7 +3576,11 @@ pub fn Renderer(comptime GraphicsAPI: type) type {
                 @intCast(cp.codepoint),
                 .regular,
                 .text,
-                .{ .grid_metrics = self.grid_metrics },
+                .{
+                    .grid_metrics = self.grid_metrics,
+                    .thicken = self.config.font_thicken,
+                    .thicken_strength = self.config.font_thicken_strength,
+                },
             ) catch |err| {
                 log.warn("error rendering preedit glyph err={}", .{err});
                 return;