Thread.svelte: Add null check for messagesContainerElement in scrollToBottom()
to match the existing pattern in Channel.svelte. Prevents potential TypeError
when the DOM element is not yet bound during rapid thread switches.
PinnedMessagesModal.svelte: Move res.length check inside the if (res) block.
Previously, res.length was accessed unconditionally after a guarded block,
causing TypeError when the API call fails and the .catch() returns null.