fix(dashboard): umlauts/dead-key input in xterm (#76233)

[webtecnica]

Closes #76233

[alt-glitch]

This was generated by AI during triage.

Related: #76257 targets the same dead-key report (#76233), but the live diffs use different fallback predicates: this patch counts only onData during composition, while #76257 defers a pending commit and cancels it when matching terminal data arrives. Please choose the intended xterm composition contract; these are not duplicates.

[teknium1]

Thanks for addressing a verified gap in the dashboard input path: current main records compositionend only for mobile replacement handling (web/src/pages/ChatPage.tsx:766-771), while PTY input is forwarded from term.onData (web/src/pages/ChatPage.tsx:1182-1212).

Problems

• web/src/pages/ChatPage.tsx:1256 treats any printable onData event during composition as proof that xterm delivered the composition, and :822 then suppresses the fallback. Unrelated printable input can therefore drop the pending composed text. The related #76257 implementation explicitly keeps pending text unless terminal data matches it.
• The diff changes only web/src/pages/ChatPage.tsx; it adds no regression coverage for the asynchronous fallback and duplicate-suppression contract.

Suggested changes

• Keep a pending composition value and cancel only on terminal data correlated to that value, rather than a count of arbitrary printable data.
• Extract and test that helper for dropped, matching, unrelated, rapid, empty, and disposed compositions.

This is an automated hermes-sweeper review.

[teknium1]

This predicate counts every printable onData payload during composition, not just the committed value from this composition. Unrelated input can make the counter nonzero and suppress the fallback at line 822, dropping ev.data; retain a pending commit and clear it only when terminal data is correlated to that pending value.