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/types/index.ts

@@ -71,6 +71,7 @@ export interface StartTimerInput {
 
 export interface UpdateTimerInput {
   projectId?: string | null;
+  startTime?: string;
 }
 
 export interface StopTimerInput {