fix(input): carry key and preedit text as bytes like Zig

Zig's KeyEvent.utf8 and the embedded API's key/preedit text are byte
slices passed through to key encoding and the pty; the port converted
them lossily to UTF-8.
- KeyEvent.utf8 is &[u8] end to end (ghostty-input encoder, core,
  ghostty-lib C API, libghostty-vt key events, GTK callers). Where Zig
  validates with Utf8View (alternates, modify-other-keys, ctrl CSI-u,
  associated text, binding unicode lookup) invalid text takes the same
  "no UTF-8" fallback.
- Invalid preedit UTF-8 behaves like Zig's `try Utf8View.init`: the
  prior preedit is cleared and marked dirty, the callback returns an
  error (logged by the C API) and no preedit is set.
- The inspector key widget shows invalid key text lossily, display only.

Test Plan:
- cargo test --workspace --no-fail-fast (3407 passed)
- cargo test -p ghostty-lib --features test-platform (C-API key text
  0xff 0xfe reaches the pty unchanged)
- just check-macos (both), just check-windows, just check-wasm

crates/ghostty-input/src/key.rs

@@ -43,7 +43,7 @@ pub struct KeyEvent<'a> {
     /// This will be an empty string if there is no text generated.
     /// If composing is true and this is non-empty, this is preedit
     /// text.
-    pub utf8: &'a str,
+    pub utf8: &'a [u8],
 
     /// The codepoint for this key when it is unshifted. For example,
     /// shift+a is "A" in UTF-8 but unshifted would provide 'a'.
@@ -58,7 +58,7 @@ impl Default for KeyEvent<'_> {
             mods: Mods::default(),
             consumed_mods: Mods::default(),
             composing: false,
-            utf8: "",
+            utf8: b"",
             unshifted_codepoint: 0,
         }
     }