Seat switcher and omniscient mode for solo testing

[jordanwallwork]

Parent: Issue #98

Toolbar controls for solo playtesting. Switch between seat perspectives or view all cards.

• Seat switcher button (only shown when seat zones exist)
• Dropdown: list 1..N seats + 'Omniscient'
• Click to change perspective
• Omniscient mode: shows all cards regardless of ownership
• Default: omniscient on setup launch
• Click seat in list: view from that seat's perspective (applies visibility masking)
• Integrates with existing visibility model

Acceptance criteria

• Seat switcher button appears when seat zones present
• Dropdown lists all seats + Omniscient
• Clicking seat changes tabletop view
• Omniscient mode shows unmasked cards
• Visibility masking applies when viewing from a seat
• Defaults to omniscient on setup launch
• Does not affect saved state (view-only)

Blocked by: #116, #118

[jordanwallwork]

Implemented in commit 47a1d44 on feature/game-setup: toolbar seat switcher (shown only when seat zones exist), dropdown of seats + Omniscient, defaults to omniscient on setup launch, applies the #116 visibility masking, view-only (no state mutation). Included in PR #125.

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