fix(activity): remove gap above selected-file detail

[tcballard]

Summary

Implements the Activity detail-layout correction for the v0.1.1 release.

Adds:

• A fitted native Activity table that grows with its rows and scrolls when space is constrained
• A selected-file detail surface placed directly beneath the table instead of below unused table canvas
• Preservation of native table sorting, selection, context menus, scrolling, and accessibility

Scope

Included

• Replace the resizable Activity VSplitView with a content-aware vertical layout
• Top-align the selected-file detail surface so unused space remains below it
• Retain the existing Activity columns and actions

Excluded

• Changes to Inbox layout
• Changes to Activity storage, retention, or sorting behavior
• Changes to filing or routing logic

Product / Architecture Decisions

• Keep SwiftUI Table as the native data surface rather than replacing it with custom rows.
• Treat the table and selected-file detail as one selection-driven hierarchy; spare window canvas belongs below both.
• Cap the table against available window height so long histories continue to scroll.

User-Facing Contract

Human Output

The selected Activity detail now appears immediately after the final visible table row. The former black band between the table and detail is removed.

Verification

Ran

• ./script/build_and_run.sh --verify
• swift test
• Rendered inspection of the installed Activity view

Covered

• Debug build and launch
• Developer ID signing for the app and Finder extension
• Finder extension registration
• 74 Swift tests
• Activity table with a populated retained history
• Native sorting, scrolling, selection, context-menu, and accessibility structure

Review Path

1. DashboardView.activityContent
2. ActivityPaneMetrics
3. Render the Activity view with retained history

Notes For Reviewer

The layout remains entirely SwiftUI and keeps the native Table; the change only revises the vertical relationship between the table, selected detail, and spare canvas.