fix(v2): rename rune stores to .svelte.ts to fix rune_outside_svelte error

Svelte 5 $state/$derived runes only work in .svelte and .svelte.ts
files. The stores had plain .ts extensions, causing a blank screen with
"rune_outside_svelte" runtime error. Renamed all three store files and
updated import paths across 5 consuming files.

v2/src/lib/components/Sidebar/SessionList.svelte

@@ -7,7 +7,7 @@
     getActivePreset,
     setPreset,
     type LayoutPreset,
-  } from '../../stores/layout';
+  } from '../../stores/layout.svelte';
 
   let panes = $derived(getPanes());
   let preset = $derived(getActivePreset());