ux: no-scroll split-army — selected-army panel under the map

[dmccoystephenson]

Summary

Fixes #51. Splitting an army required scrolling past the map, status, policy, and settings panels to the Armies list at the bottom, then finding the army by #id. This brings the action to where the player already is: a compact "selected army" panel directly under the map that acts on the army you clicked.

Round — bring army actions to the map

1. Panel under the map — a compact selected-army panel is added right below the canvas; by default it shows a hint ("Click one of your armies on the map to select it") to make the select-then-act flow discoverable (Nielsen Implement Enhanced UI with Mouse Controls and HUD #7, Implement Basic Rule-Based AI Opponent #6).
2. Driven by the map selection — the panel refreshes on every selection change and tick, showing "Army #N — S soldiers at (x, y)", and falls back to the hint when nothing is selected or the army no longer exists (Nielsen Implement Basic Rule-Based AI Opponent #6).
3. Split from the panel — the panel's Split button validates against the selected army's soldier count and issues the SPLIT command, so no scrolling is needed; the request is factored into a shared sendSplitCommand() used by the list too, and now redirects to login on a 401 like the other actions.
4. Split button off the destructive red — Split Army was the same red as Delete/Logout; recolored to the neutral action brown with a proper disabled state (Nielsen Implement Army Management (Split, Merge, Display) #4).

Test plan

• Click one of your armies on the map → the under-map panel shows its stats and a split control (no scrolling).
• Enter an amount and click Split → the army splits; a success toast appears; the panel reflects the new soldier count.
• Select an army with 1 soldier → the split control is disabled with a "Needs 2+ soldiers" hint.
• Right-click / move away → the panel returns to the hint.
• The bottom Armies-list split still works; both Split buttons are neutral-colored, not red.

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