App navigation: file creation actions in waffle menu

[pringelmann]

Sub-issue of #59888

Add a sticky bottom row to the waffle popover with file creation shortcuts (Document, Spreadsheet, Presentation) plus an "All types" entry that opens Files' "Create new" section in-place.

The architecture is the open question. The first instinct was to redirect into Files for the actual creation, but Files' new-file menu actions need a folder context to evaluate enablement, and reusing them from outside Files would Frankenstein the API and likely break app developers we promised not to break. So the right answer is a new core API for header-mounted actions:

• Apps register an action declaring what context it needs (e.g. a folder)
• The API guides the user through obtaining that context
• The action only runs once the context resolves

This is a sizable new API, not a small wrapper. Probably 2+ weeks once it's properly specced. Worth scoping the design with broader team before starting implementation.

Not in scope: Office MIME-icon coordination (parallel work, separate timeline) and apps other than Files registering actions in this initial pass.