UI: update notice, Help › Check for Updates and Reinstall, Settings › Updates (#231)

The window checks at start-up at most once a day when automatic checks are on (Settings and
the first run's question), never while that question is open and never without a host, so the
browser and tests make no network call. A newer release shows a quiet notice in the status bar
("FieldCraft X is available — What's new · Install · Skip"); Install downloads and verifies
through the host, then asks to restart, saving or asking about unsaved changes first, and the
host swaps the release in after the window closes. A .deb/.rpm install offers the system's
installer and the command instead. Skip this version is kept in the preferences.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/ui-egui/Cargo.toml

@@ -15,6 +15,8 @@ fieldcraft-render = { workspace = true }
 fieldcraft-explicit = { workspace = true }
 fieldcraft-geometry = { workspace = true }
 fieldcraft-decks = { workspace = true }
+# Releases and staged installs, for the update notice (the host does the downloading).
+fieldcraft-update = { workspace = true }
 egui = { workspace = true }
 craft-ui = { workspace = true }
 egui-wgpu = { workspace = true }