feat: implement My Account page

Add /account route showing display name (from localStorage), role badge,
session expiry decoded from JWT, and recovery PIN display with copy button.
Join and recover flows now persist display_name to localStorage via setAuth().
Feed header logout button replaced with person-icon link to /account;
logout is available from the account page.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/join/+page.svelte

@@ -22,7 +22,7 @@
 				is_new: boolean;
 			}>('/join', { display_name: displayName.trim() });
 
-			setAuth(res.jwt, res.pin, res.user_id);
+			setAuth(res.jwt, res.pin, res.user_id, displayName.trim());
 			pin = res.pin;
 			showPinModal = true;
 		} catch (e) {