Fix typed text never reaching shared drafts

[lsegal]

Typing in a shared draft never reached the other browser, in either direction.

Root cause

The screenplay page is one contentEditable host (.screenplay-editable), and the paragraphs inside it are plain children. Browsers send input events to the editing host, not to the paragraph being typed in, so the per-paragraph onInput handler never ran. Typed characters stayed in the DOM and were only written to the Yjs document when a key handler happened to capture the paragraph text (Enter, Tab, Delete…). Nothing reached the shared doc, so y-webrtc had nothing to send. As a side effect, plain typing was also lost on reload.

I reproduced this with two isolated Chrome contexts against this repo's own signaling worker (wrangler dev). The peers connected ("2 in room") and the initial document state synced, but typed text never reached the other side, or even the local Yjs doc.

Change

• Handle input on the .screenplay-editable host, resolve the edited paragraph from the event target or selection (shared with the existing keydown lookup), and call project.updateParagraph. Removed the dead per-paragraph onInput.
• Record the caret offset before the update so the existing pendingCaret layout effect puts it back after React re-renders the paragraph text. Without this, each keystroke would reset the caret to the start.

Verification

• New app/src/App.test.tsx: mounts the editor, sends an input event to the editing host the way a browser does, reopens the draft from IndexedDB, and checks that the text persisted. It fails on main (expected [ '' ] to include 'INT. KITCHEN - NIGHT') and passes with this change.
• pnpm run lint and pnpm run test (38 tests) pass locally.
• Two-browser check (local signaling worker): A typed FROMA, then B appended FROMB. Both ended with FROMAFROMB. Mid-text insertion keeps the caret in place, and text survives a reload.

Browser B after both edits (A's presence pointer visible):

Browser A after both edits:

Still needed on write.subscript.to (deployment, not code)

The hosted site uses the default signaling URL wss://y-webrtc-eu.fly.dev (the <subscript-write> embed sets no signaling-url). That server no longer responds; connections time out. Until the embed points at a working signaling server (for example a deployed signaling/ worker), browsers on write.subscript.to still can't find each other. A follow-up issue will be filed for this when this PR merges.

Changelog

No changelog entry: the project has no changelog.

Closes #1

Agents: claude-code (claude-opus-5-5)

[lsegal]

Merged. Follow-ups filed:

• Hosted sharing still fails: default signaling server wss://y-webrtc-eu.fly.dev is offline #7: write.subscript.to still can't connect peers, because its default signaling server wss://y-webrtc-eu.fly.dev is offline. This PR fixes edit sync, but hosted sharing stays broken until Hosted sharing still fails: default signaling server wss://y-webrtc-eu.fly.dev is offline #7 is done.
• Fresh draft gains an extra empty paragraph after first reload #8: a fresh draft gains an extra empty paragraph after the first reload (pre-existing).

Manual verification

CI can't cover these: they need real deployed infrastructure and separate devices.

• After Hosted sharing still fails: default signaling server wss://y-webrtc-eu.fly.dev is offline #7 is deployed, open a draft on https://write.subscript.to on one device and click Share Draft. Open the link on a second device on a different network (e.g. a phone on cellular). Check that both show "2 in room", that typing on either side shows up on the other within a few seconds, and that the caret stays where you are typing while the other side edits.