Assign readable ticket keys from project code and sequence.

Replace UUID-looking sync labels in TUI and web with keys like TAS-1,
while keeping the remote UUID for task-master matching.

Co-authored-by: Cursor <cursoragent@cursor.com>

web/src/App.svelte

@@ -907,7 +907,7 @@
                     >
                       <button type="button" class="ticket-open" onclick={() => onBoardTicketClick(t)}>
                         <div>{t.title}</div>
-                        {#if t.externalId}<div class="muted">{t.externalId}</div>{/if}
+                        {#if t.key || t.externalId}<div class="muted">{t.key || t.externalId}</div>{/if}
                       </button>
                       <span class="drag-handle" title="Ziehen" aria-hidden="true">⠿</span>
                     </div>
@@ -924,6 +924,9 @@
               <button onclick={() => openProject(projectId, projectName)}>← Board</button>
               <button class="danger" onclick={removeTicket}>Delete</button>
             </div>
+            {#if selected.key || selected.externalId}
+              <div class="muted">{selected.key || selected.externalId}</div>
+            {/if}
             <label>Title<input bind:value={editTitle} /></label>
             <label>Status
               <select bind:value={editStatus}>