feat(settings): runtime toggle for prefer-window-name

[0x79H]

Fixes #164.

What

AGENTBOARD_PREFER_WINDOW_NAME currently requires a server restart and isn't discoverable. This wires it into the existing runtime-settings pattern (like tmux-mouse-mode):

• GET/PUT /api/settings/prefer-window-name, persisted under prefer_window_name in app_settings; the env var remains the initial default, a stored value wins once toggled (same contract as history-max-age-hours). Persist failure rolls back the runtime value and returns 500 (same contract as tmux-mouse-mode).
• The session refresh worker receives the value per request (RefreshWorkerRequest.preferWindowName) since its own config snapshot is frozen at spawn time.
• PUT rebroadcasts server-config so already-connected clients (mobile tab labels read preferWindowName from the session store) converge without a reload, and schedules a refresh so sidebar names update within about a second.
• Settings modal: a "Prefer Window Names" switch placed next to Mouse Mode (the immediate-apply section, not the draft section, so Cancel semantics stay honest).

Verification

• Container (Debian trixie, non-root, HOSTNAME=127.0.0.1): full suite green, coverage mode and vite build pass on this branch in isolation.
• Manually exercised end-to-end on a live deployment: toggling flips sidebar labels between session names and window names within one refresh tick, persists across restarts, and the env var still seeds the initial default.

Feel free to rework or take this over entirely — happy either way.