Standardize way for affiliations

Bug description

Hello Quarto devs!

My understanding is that, for articles, we specify affiliations like this (reference):

author:
  - name: Norah Jones
    affiliation:
      - ref: cmu
      - ref: chicago
  - name: John Hamm
    affiliation:
      - ref: cmu
affiliations:
  - id: cmu
    name: Carnegie Mellon University
  - id: chicago
    name: University of Chicago

While for beamer presentations we have to use (reference):

author: 
  - John Doe\inst{1}
  - John Roe\inst{2}
institute: 
  - \inst{1} affiliation for John Doe
  - \inst{2} affiliation for John Roe

Otherwise the affiliations will not appear in the title slide.

The latter syntax however causes the affiliation to be displayed in the bottom left corner.

I propose to standardize the way we specify affiliations so it's always the same across the two backends.

Related #1658

Steps to reproduce

See above

Expected behavior

No response

Actual behavior

No response

Your environment

No response

Quarto check output

➜  quarto check
Quarto 1.5.57
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.2.0: OK
      Dart Sass version 1.70.0: OK
      Deno version 1.41.0: OK
      Typst version 0.11.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.5.57
      Path: /opt/quarto/bin

[✓] Checking tools....................OK
      TinyTeX: (not installed)
      Chromium: (not installed)

[✓] Checking LaTeX....................OK
      Using: Installation From Path
      Path: /usr/bin
      Version: undefined

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.12.5
      Path: /usr/bin/python3
      Jupyter: 5.7.2
      Kernels: ir, python3

(\) Checking Jupyter engine render....[IPKernelApp] WARNING | Config option `autoindent` not recognized by `ZMQInteractiveShell`.  Did you mean `autoawait`?
[✓] Checking Jupyter engine render....OK

Using 18 cores for parallelisation
[✓] Checking R installation...........OK
      Version: 4.4.1
      Path: /usr/lib64/R
      LibPaths:
        - /home/raffaele/R/x86_64-pc-linux-gnu-library/4.4
        - /usr/lib/R/library
      knitr: 1.48
      rmarkdown: 2.28

[✓] Checking Knitr engine render......OK

Reproduce the article and Beamer examples from the issue using the linked front-matter documentation and the Quarto check environment. Compare how author affiliations are rendered on each title slide, then define completion as one consistent affiliation syntax working across both backends without unwanted placement changes.