fix(app): preserve IME composition across text fields (#3517)

* fix(app): preserve IME composition in form inputs

Form model publication was replaying changing default text into the native or DOM editing surface. Keep the editing surface authoritative, defer web publication until composition commits, and reserve resetKey changes for explicit text replacement.

* refactor(app): centralize text editing authority

Route every app text field, including the composer and plugin runtime, through one platform-owned editing primitive. Its type and runtime reject controlled value replay, its command handle owns programmatic replacement, and lint prevents raw React Native TextInput imports outside the boundary.

* fix(app): restore composer placeholder styling

docs/forms.md

@@ -61,6 +61,12 @@ complete`), keyed off the opened snapshot's serverId. Waiting for data is a
   states) is owned by `components/ui/control-geometry.ts` — controls never
   declare their own, and screens never nudge global component styles to align
   a row.
+- Every text field consumes `EditingTextInput`, directly or through a UI
+  wrapper. The editing surface owns in-progress text; React observes committed
+  edits through `onChangeText`, and programmatic mutations use the input's
+  `replaceText` command. Its props omit `value` and `defaultValue`, web IME
+  candidates stay unpublished until composition commits, and lint rejects raw
+  React Native `TextInput` imports outside the primitive.
 - The form declares one size for all fields: `sm` on desktop, `md` compact
   (`useIsCompactFormFactor`).
 - Availability hierarchy: a field whose capability doesn't apply is **hidden**