fix(web): coerce chat composer draft to string (input.trim crash)

The Send button's disabled={!input.trim()} threw "trim is not a function" when
input was initialized from a non-string initialDraft (a Svelte 5 prop-init edge
where a null/undefined draft reached $state). Coerce at init so the composer
state is always a string.

VERSION: 0.15.0 -> 0.15.1

VERSION

@@ -1 +1 @@
-0.15.0
+0.15.1

web/src/lib/components/ChatThread.svelte

@@ -58,7 +58,7 @@
     initialDraft?: string
   } = $props()
 
-  let input = $state(initialDraft)
+  let input = $state(typeof initialDraft === 'string' ? initialDraft : '')
   let messagesEnd = $state<HTMLDivElement | null>(null)
   let scrolledUp = $state(false)
   let container = $state<HTMLDivElement | null>(null)