Add 'f' shortcut to open changelist filters

[Quentin-Jadoul]

Description
This PR adds a keyboard shortcut on changelist pages to open the filters panel.

Changes

• Added a changeList() Alpine helper in src/unfold/static/unfold/js/app.js
• Added an f shortcut to open filters
  • works case-insensitively
  • ignored when typing in inputs, textareas, or contenteditable elements
  • only active when filters are available (cl.has_filters)
• Wired the changelist root container to changeList(...) in src/unfold/templates/admin/change_list.html
• Added an f shortcut hint next to the Filters button
• Kept existing Esc behavior for closing filters

Why

• Makes filters quicker to open on changelist pages
• Keeps shortcut behavior aligned with existing shortcuts like t for search

Notes

• Only applies to changelist filters
• No changes outside changelist pages