fix: give the rich text editor an accessible name (WCAG 4.1.2, 3.3.2) (#27503)

On latest `dev`, `RichTextInput` passes only `attributes: { id }` to tiptap, so the rendered contenteditable has an implicit `textbox` role and **no accessible name at all**.

The only label is the tiptap placeholder, which renders as CSS generated content in `src/app.css` via `content: attr(data-placeholder)`. Generated content never becomes an element's accessible name, so assistive technology announces the field as "edit text, blank".

This is the chat composer, the channel and thread composers, and the note editor, so it is the most used control in the product.

Breaks WCAG 4.1.2 Name, Role, Value (Level A), and 3.3.2 Labels or Instructions (Level A), since the only instruction is invisible to assistive technology.

Fix: expose the placeholder as `aria-label` on the editor element.

`attributes` is passed as a **function** rather than an object literal. The object form is evaluated once when the `Editor` is constructed and never rebuilt, but `placeholder` is deliberately runtime mutable: `channel/MessageInput.svelte` and `channel/Thread.svelte` swap it between "You do not have permission to send messages in this thread." and "Reply to thread..." once `channel` resolves, and it also changes when the interface language changes. With the object form the field would have been permanently named with whatever string happened to be set at mount, which for a channel the user *can* write to is the no-permission message. That would be worse than no name at all. ProseMirror supports the function form and re-evaluates it on every state update, and the component's existing `setPlaceholder` already dispatches an empty transaction, so the label now tracks the visible placeholder. It binds to `_placeholder`, the same value that feeds the visible text, so the two cannot diverge.

`aria-multiline` is deliberately not set. It is only valid on an explicit `textbox`/`searchbox` role, and adding `role="textbox"` would flatten the editor's inner structure so headings, lists and links inside rich text stop being exposed.

Severity: Critical. The application's primary input announces as an unnamed edit field.

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src/lib/components/common/RichTextInput.svelte

@@ -965,7 +965,9 @@
 				}
 			},
 			editorProps: {
-				attributes: { id },
+				// the tiptap placeholder never becomes the field's accessible name;
+				// function form so a placeholder change is picked up after mount
+				attributes: () => ({ id, 'aria-label': _placeholder }),
 				handleDrop: (view, event) => {
 					// Intercept sidebar chat item drops to prevent ProseMirror
 					// from inserting the raw JSON as text. The actual handling