fix: Enter submits during Japanese/CJK IME composition

[wmeddie]

Summary

• Pressing Enter during Japanese (and other CJK) IME composition was sending the message / triggering the input's action instead of confirming the IME candidate.
• Root cause: on macOS's built-in Japanese IME, the Enter keydown that confirms a candidate can fire after compositionend with e.isComposing === false and e.keyCode === 13, so guarding only on e.isComposing or keyCode === 229 is insufficient.
• Fix: every Enter-to-submit input now tracks composition via oncompositionstart / oncompositionend, with the flag cleared via setTimeout(..., 0) so the confirming Enter keydown that may follow still sees composition as active.

Scope

• Chat / message inputs (full pattern): homepage, conversation page, task page.
• Other Enter-triggered inputs (full pattern): agent memory search, global memory search, agent tool catalog search, agent task title, workspace volume mount, setup workspace folder.
• VariablePopup only receives a KeyboardEvent from its parent, so it gets the lighter !e.isComposing && keyCode !== 229 check rather than local state.

Test plan

• svelte-check clean (0 errors)
• Tested in the Tauri desktop app with the macOS built-in Japanese IME — typing hiragana and pressing Enter to confirm a candidate no longer sends the message; a second Enter still sends as expected.
• Regression: Shift+Enter still inserts a newline in chat inputs; plain Enter still sends when not composing.
• Re-verify on other IMEs if available (Chinese / Korean).