docs(command-zone): add developer and user documentation

Add comprehensive documentation for the command zone feature.

Developer documentation:
- command_zone_architecture.md: detailed architecture overview
  - Layer separation (types, state, logic, graphics)
  - Component interactions and dependencies
  - Testing strategy and coverage
- Index update to include new architecture doc

User documentation:
- commander_format.md: how to use Commander format in Cockatrice
  - Zone configuration options
  - Gameplay mechanics (commander tax, zone reveals)
  - Tips for Commander games
- Index update to include new format doc

doc/doxygen/extra-pages/user_documentation/formats/commander_format.md

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+@page commander_format Commander Format Support
+
+# Overview
+
+Cockatrice provides dedicated support for the Commander (EDH) format, including specialized zones for managing your commander and partner commander cards.
+
+## What is the Command Zone?
+
+The Command Zone is a special area where your commander resides when not on the battlefield. In Cockatrice, this is represented as a dedicated zone that appears above the stack when playing Commander format games.
+
+Key features:
+- **Primary Command Zone**: Holds your main commander card
+- **Partner Zone**: Expandable secondary zone for partner commanders
+- **Companion Zone**: Optional zone for Ikoria companion cards
+- **Background Zone**: Optional zone for Baldur's Gate background enchantments
+- **Commander Tax Counters**: Visual tracking of commander tax (the additional mana cost each time you cast your commander from the command zone)
+
+## Enabling the Command Zone
+
+The Command Zone automatically appears when the game server indicates you're playing a Commander format game. If you don't see the Command Zone, check that:
+1. The game room is configured for Commander format
+2. The server supports command zones
+
+## Using the Command Zone
+
+### Designating a Commander
+
+To place a card in the Command Zone:
+1. Drag the card from your hand, library, or other zone to the Command Zone
+2. Primary and Partner zones accept one card each; Companion and Background zones can hold multiple cards (for edge-case mechanics like Rule 0 multiple companions)
+
+### Zone States
+
+All zones toggle between **full size** and **minimized (25% height)** via double-click. Tax counters remain visible when minimized.
+
+### Partner Zone
+
+Click the toggle button at the bottom of the Command Zone (or use the context menu) to expand/collapse the Partner Zone. It behaves identically to the primary zone with its own card display, minimize capability, and dedicated Partner Tax counter.
+
+> **Note**: The Partner Zone will not collapse if it contains a card or has a tax counter value greater than 0. This prevents accidentally hiding important game state.
+
+### Companion Zone
+
+The Companion Zone is designed for the Ikoria companion mechanic:
+
+**When to use**: If your deck includes a companion card (a creature that can start outside the game if your deck meets its restriction), place it in the Companion Zone.
+
+**Enabling**: The Companion Zone must be enabled when creating the game (see Game Creation Options below).
+
+**Features**:
+- Displays your companion card(s) publicly (supports multiple for Rule 0 edge cases)
+- No tax counter (companion mechanic doesn't use commander tax)
+- Collapsible to save screen space
+
+### Background Zone
+
+The Background Zone is designed for the Baldur's Gate "Choose a Background" mechanic:
+
+**When to use**: If your commander has "Choose a Background" (like many commanders from Commander Legends: Battle for Baldur's Gate), place your background enchantment in this zone.
+
+**Enabling**: The Background Zone must be enabled when creating the game (see Game Creation Options below).
+
+**Features**:
+- Displays your background enchantment(s) publicly (supports multiple for edge cases)
+- No tax counter (backgrounds don't have commander tax)
+- Collapsible to save screen space
+
+## Commander Tax Tracking
+
+Cockatrice tracks commander tax with dedicated counters:
+
+- **Commander Tax Counter**: Appears in the top-left of the Command Zone
+- **Partner Tax Counter**: Appears in the top-left of the Partner Zone (when expanded)
+
+### Adjusting Tax Counters
+
+Using the context menu (right-click on the zone):
+- "Increase Commander Tax Counter (+1)" - Increments the tax by 1
+- "Decrease Commander Tax Counter (-1)" - Decrements the tax by 1
+
+Tax counters track the number of times your commander has been cast during the game.
+
+## Context Menu Options
+
+The battlefield's "Commander" submenu provides consolidated access to all zone actions: view zones, adjust tax counters (+1/-1), and toggle partner/companion/background zones. Right-clicking directly on a zone shows zone-specific options (view, tax counter for Primary/Partner, toggle, minimize).
+
+## UI Summary
+
+| Element | Location | Action |
+|---------|----------|--------|
+| Command Zone | Above the stack | Holds primary commander |
+| Partner Zone | Below Command Zone (when expanded) | Holds partner commander |
+| Companion Zone | Below Partner Zone (when enabled) | Holds companion card(s) |
+| Background Zone | Below Companion Zone (when enabled) | Holds background enchantment(s) |
+| Toggle Button | Bottom of Command Zone | Click to expand/collapse Partner Zone |
+| Tax Counter | Top-left of Primary/Partner zones | Shows current commander tax |
+| Minimize | Zone background | Double-click to toggle 25% height mode |
+
+## Game Creation Options
+
+When creating a game, you can configure which zones are available:
+
+| Option | Default | Description |
+|--------|---------|-------------|
+| Enable Command Zone | Unchecked | Enables commander and partner zones with tax counters |
+| Enable Companion Zone | Unchecked | Enables the Companion Zone for Ikoria companion mechanic |
+| Enable Background Zone | Unchecked | Enables the Background Zone for Baldur's Gate backgrounds |
+
+All three options are independent checkboxes. You can enable any combination:
+- **Command Zone only**: Standard Commander/EDH gameplay
+- **Companion Zone only**: Use companion cards in any format
+- **Background Zone only**: Use backgrounds without commander zones
+- **Any combination**: Mix and match as your game requires
+
+**Defaults for new games**: All zone options default to unchecked. Your preferences are saved for future games when "Remember settings" is enabled.
+
+## Tips
+
+- Tax counters are persistent throughout the game and visible to all players
+- Cards in the Command Zone are visible to all players (public information)
+
+## See Also
+
+- @ref creating_decks - How to create and edit decks
+- @ref editing_decks - Detailed deck editing guide