feat(observer): add observer mode and disconnect recovery

docs/OBSERVER_UI.md

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+# Observer browser mode
+
+Milestone 009 adds a password-free observer role to the same local browser
+application. The role is still authenticated by its temporary session token,
+but joining it does not request a player name or consume either player slot.
+At most four observer sessions are connected at once.
+
+## Projection and authorization
+
+An observer renders only the server's `observer` view projection. That
+projection contains all maze cells, remaining objects, both player positions,
+inventories, effects, exit state, phase, and result. The browser does not
+simulate movements or infer updates. Each player status also reflects the
+server-owned WebSocket connection state. A complete projection establishes the
+server-approved static geometry on join, restoration, and match start.
+Subsequent bounded projections replace all dynamic player and object state
+while reusing only that authoritative geometry, avoiding an 8,192-cell
+retransmission after every move.
+
+The observer screen contains no movement, start, or drop-key control. This is
+only presentation-level defense: the gateway also rejects and logs observer
+`start`, `move`, and `drop_key` attempts before they reach authoritative
+mutation. Disconnecting an observer changes only lobby presence and never
+pauses the game.
+
+## Viewport and navigation
+
+At `1024 x 768` CSS pixels and above, the complete `128 x 64` default map is
+initially scaled to fit the available stage. The observer can zoom with the
+labelled buttons or mouse wheel and pan with pointer dragging. Zoom is bounded
+between 0.25 and 6 times the eight-pixel base cell size; the **Вся карта**
+control restores the calculated fit and centered position.
+
+When either viewport dimension is below the supported minimum, the map and
+controls are hidden and the Russian message **Нужен экран побольше** explains
+the `1024 x 768` requirement. All assets remain local and use the shared
+retro-arcade visual language.