fix: align plugin install progress

[beallio]

Please tick as appropriate:

• I have tested this code on a Steam Deck or on a PC
• My changes generate no new errors/warnings
• This is a bugfix/hotfix
• This is a new feature

Description

Moves plugin installation progress out of the dialog title and into a shared, full-width progress panel below the description.

Previously, the progress widget used an inline, fixed-width layout that pushed the operation text to the right and caused the progress bar to overflow or shift as the text changed.

The new progress panel:

• Spans the modal content width, aligning with the outer edges of the dialog buttons.
• Left-aligns the operation text.
• Keeps the progress bar within the panel’s standard padding.
• Uses Decky’s native Field and ProgressBar components.
• Shares the same layout between single-plugin and multi-plugin installations.

This fixes issue: #943