Improve agent context controls and notice readability

[DevMando]

Summary

• Use a list-style Files icon and make the composer placeholder identify the active agent, not the application.
• Add Desktop support for /compact, persist compacted session context, and make /clear explicitly state that it wipes context completely.
• Render recovery, plan, approval, and general status notices as high-contrast glass cards so they remain readable when conversation backgrounds use high image opacity. Existing connected/status pills keep their color-dot treatment.
• Update the Desktop core reference to include the shared planner and compaction behavior.

Why this matters

Agent context actions are clearer and safer, while important progress and recovery messages remain legible across all appearance themes, including image-heavy themes.

Dependency and merge order

Depends on DevMando/MandoCode#84. Merge that core/CLI PR first, then merge this Desktop PR.

Validation

• dotnet build src/MandoCode.Desktop/MandoCode.Desktop.csproj --no-restore (0 warnings, 0 errors)