feat: add account deletion feature (#85)

* feat: add account deletion feature

- Add DELETE /auth/me endpoint to delete user account and all data
- Add deleteAccount() method to AuthContext
- Add Delete Account button with confirmation modal in UserProfile
- Add danger zone styling (.btn-danger, .profile-danger-zone)
- Add i18n translations for EN and DE
- Add backend tests for account deletion endpoint
- Add timeout settings to frontend vitest.config.ts
- Reduce CI timeout for frontend tests (10min -> 5min)

* fix: improve delete account section layout

- Make profile modal scrollable with max-height
- Add proper horizontal margin to danger zone
- Align delete section with form content

* fix: use ConfirmModal component for delete account dialog

- Replace inline modal with existing ConfirmModal component
- Ensures consistent button styling across all modals
- Add UI consistency rule to AGENTS.md and copilot-instructions.md

* fix: consistent styling for delete account section

- Remove warning text (users know what delete means)
- Remove border-bottom from danger zone title (section has border-top)
- Update copilot-instructions and AGENTS.md with stricter UI consistency rules
- Remove unused deleteAccountHint i18n keys

* chore: remove pre-push test hook (CI handles tests)

Tests were running twice - in pre-push hook and GitHub CI.
Removing local pre-push tests since CI provides authoritative test results.
Use 'npm test' manually before pushing if you want local feedback.

frontend/src/styles.css

@@ -4182,7 +4182,8 @@ h3 .reminder-icon.info-tooltip {
 .profile-modal {
 	max-width: 420px;
 	padding: 0;
-	overflow: hidden;
+	overflow-y: auto;
+	max-height: 90vh;
 }
 
 .profile-container {
@@ -4366,6 +4367,39 @@ h3 .reminder-icon.info-tooltip {
 	cursor: not-allowed;
 }
 
+/* Profile danger zone */
+.profile-danger-zone {
+	margin: 0 1.5rem 1.5rem;
+	padding-top: 1.5rem;
+	border-top: 1px solid var(--border-primary);
+}
+
+.profile-danger-zone .profile-section-title {
+	border-bottom: none;
+	padding-bottom: 0;
+	margin-bottom: 0.75rem;
+}
+
+.btn-danger {
+	background: #dc2626;
+	color: white;
+	border: none;
+	padding: 0.5rem 1rem;
+	border-radius: 6px;
+	font-weight: 500;
+	cursor: pointer;
+	transition: background 0.15s ease;
+}
+
+.btn-danger:hover:not(:disabled) {
+	background: #b91c1c;
+}
+
+.btn-danger:disabled {
+	opacity: 0.5;
+	cursor: not-allowed;
+}
+
 /* =============================================================================
    About Modal
    ============================================================================= */