Add ability to manually adjust the running timer's start time

Allows users to retroactively correct the start time of an ongoing timer
without stopping it. A pencil icon in the timer widget opens an inline
time input pre-filled with the current start time; confirming sends the
new time to the backend which validates it is in the past before persisting.

backend/src/schemas/index.ts

@@ -64,6 +64,7 @@ export const StartTimerSchema = z.object({
 
 export const UpdateTimerSchema = z.object({
   projectId: z.string().uuid().optional().nullable(),
+  startTime: z.string().datetime().optional(),
 });
 
 export const StopTimerSchema = z.object({