Replace custom JS toast system with HTMX-native patterns

[chriscarrollsmith]

Summary

• Toast migration: Replaced the custom showToast() JS function and client-side flash cookie reader with server-side rendering via htmx-ext-remove-me for auto-dismiss, HTMX responseHandling config for error-status OOB swaps, and a flash cookie middleware for full-page-load toasts.
• Avatar update fix: Replaced HX-Refresh: true on avatar upload with OOB swaps for both the profile display and navbar avatar. This eliminates the visible flicker from full page reloads and ensures the "Profile updated successfully" toast always appears (previously lost during refresh).
• Navbar avatar partial: Extracted the navbar avatar into a reusable partial (navbar_avatar.html) with an OOB variant (navbar_avatar_oob.html) for HTMX responses.

Test plan

• All 362 non-browser unit/integration tests pass
• All 7 Playwright browser tests pass, including:
  • test_htmx_success_toast_appears — name-only profile update shows toast
  • test_avatar_update_toast_appears — avatar update shows toast
  • test_avatar_update_no_full_reload — avatar update uses OOB swaps, no page reload
  • test_htmx_error_toast_appears — error responses show toast via OOB
  • test_flash_cookie_toast_appears — flash cookies render server-side on load
  • test_toast_auto_dismisses — toasts auto-dismiss after 5s via remove-me
  • test_toast_close_button — manual close button works

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