Enhance session sidebar with PR worktree support (#342)

* feat: refactored sessions sidebar to be workspaces centric

- Add createWorktreeOnly helper and status checks
- Add createWorktreeOnly to create a standalone worktree for the active project
- Show error toasts when no active project or not a Git repository
- Refresh session list and show success toast with created worktree details

* feat: show attention indicator and status marker in SessionSidebar

- Show unread attention indicator for sessions with pending updates
- Display status marker combining streaming and unread indicators in the session row
- Introduce attention diamond pulse animation and associated CSS for the new indicator

* fix(SessionSidebar): reserve header actions space when repo state is known

* chore(doc): add PR status colors to custom themes

docs/CUSTOM_THEMES.md

@@ -82,6 +82,13 @@ OpenChamber supports user-defined themes. Drop a JSON file into the themes direc
       "infoBackground": "#205EA620",
       "infoBorder": "#205EA650"
     },
+    "pr": {
+      "open": "#A0AF54",
+      "draft": "#878580",
+      "blocked": "#DA702C",
+      "merged": "#8B7EC8",
+      "closed": "#D14D41"
+    },
     "syntax": {
       "base": {
         "background": "#1C1B1A",