Add accordion minimal indicator

[frecano]

(My apologies for the single commit I read the discussion about this and felt like doing it and didn't look to the contributing guide until just now)

Summary

• Adds visual overlay indicators for accordion containers, inspired by
  https://github.com/AdamWagner/stackline which was discussed in Implement stack layout #24

• Shows app icons for each window in an accordion stack, with the focused window highlighted
• Indicators are clickable to switch between windows
• Adds optional vertical navigation flag so up/down always navigates accordion windows regardless of
  orientation

Configuration

New [accordion-indicator] TOML config section:

  [accordion-indicator]
  enabled = true  # Enable the indicator overlay (default: false)
  icon-size = 30   # Size of each app icon in pixels (default: 30)
  icon-padding = 2   # Space between icons in pixels (default: 2)
  bar-padding = 4   # Padding around the indicator bar in pixels (default: 4)
  position = 'left' # Position of the indicator relative to the accordion container (possible values: 'left', 'right', 'top', 'bottom'
  vertical-navigation = true   # Use up/down keys to navigate accordion windows regardless of orientation (default: false)

Changes

• New file: Sources/AppBundle/ui/AccordionIndicator.swift — overlay panel manager, SwiftUI view, config
  struct, and config parser
• Config.swift — added accordionIndicator field
• parseConfig.swift — added accordion-indicator parser entry
• refresh.swift — calls indicator refresh after layout, hides indicators when server is disabled
• TilingContainer.swift — added matchesDirection helper for vertical navigation support
• TreeNodeEx.swift — updated closestParent to use accordion-aware direction matching
• FocusCommand.swift / SwapCommand.swift / MoveCommand.swift — use accordionFocusOffset for vertical
  navigation support