Enhancement: Redesign and resize the app sidebar

[kylejschultz]

Goal

Give Prism a more intentional, flexible app sidebar that supports comfortable resizing and a cleaner navigation/library layout.

Scope

• Redesign the sidebar layout, hierarchy, spacing, and visual treatment as part of the broader app-shell facelift.
• Let users resize the sidebar within sensible minimum and maximum bounds; preserve their chosen width across restarts.
• Ensure long playlist and library labels remain readable at practical sidebar widths, using wrapping or expanded width where appropriate rather than truncating essential names.
• Revisit playlist presentation and interactions in the redesigned sidebar, including the drag-and-drop work deferred from Feature: Add music to playlists from library views #59.
• Maintain clear active states, keyboard/focus behavior, responsive behavior, and accessible resize affordances.

Acceptance criteria

• The sidebar is visually cohesive with the refreshed Prism app shell and navigation model.
• Users can resize it smoothly and its width persists after restart.
• Minimum and maximum widths prevent unusable navigation or excessive layout loss.
• Playlist and library labels remain understandable without avoidable truncation.
• Deferred sidebar playlist interactions from Feature: Add music to playlists from library views #59 are deliberately designed for the new layout, not added as a bolt-on.
• Desktop and narrow-window behavior remain usable and accessible.

Relationship to existing work

• Follow-up to Feature: Add music to playlists from library views #59, which deferred sidebar drag-and-drop until this facelift.
• Distinct from feat: make Home a full Now Playing experience #70: that issue redesigns Home as a full Now Playing experience.

Out of scope

• Reworking playback behavior or the Home/Now Playing surface itself.

[kylejschultz]

Completed through the sidebar work shipped in v0.4.0 / PR #83: the left sidebar is resizable and reorganized, and playlist actions are available through the popout. Playlist drag-and-drop is intentionally deferred and can return as a separate issue if it becomes a priority.