feature : multi-profile pooling automatically — pick fastest outbound across all active.

[Sifat-mahmud]

… profiles, auto-switch on failure

• ProfileDao: dropped single-active exclusivity in insert/edit/deleteById; added watchActiveProfiles() (plural) and getActiveProfiles().
• ProfileRepository: added setActive(id, value) and watchActiveProfiles().
• active_profile_notifier.dart: new ActiveProfiles Riverpod notifier (plural); singular ActiveProfile kept for backward-compat (ChainProfileNotifier, etc.).
• ProfilesNotifier: added toggleActiveProfile(id, value) for the new per-profile toggle UI; updated deleteProfile to use the plural list.
• merged_config_builder.dart (NEW): pools every active profile's real outbounds (prefixed ${profileId}::${tag}), strips per-profile selector/urltest/balancer groups, appends a single top-level urltest group tagged 'select'. The core's URLTest machinery auto-picks the lowest-delay outbound across ALL active profiles.
• ConnectionRepository: connect/reconnect now take List, build the merged config, and pass _merged.json to singbox.start/restart. Added mergedConfigBuilder dependency.
• ConnectionNotifier: _connectThrottled reads activeProfilesProvider and bails if empty; the auto-reconnect listener now compares SETS of profile ids and only reconnects when the active set changes.
• Updated all callers of reconnect (connection_button.dart, connection_wrapper.dart, config_option_notifier.dart, profile_notifier.dart) to pass lists.
• ProfileTile: body-tap now calls toggleActiveProfile(id, !active) — toggles on/off without deactivating others; removed auto-pop-on-success.
• HomePage: shows a 'N active profiles pooled' badge when >1 are active.
• ActiveProxyFooter: strips the ${profileId}:: prefix from the displayed tag and shows 'via ' underneath.
• auto_profile_switch_notifier.dart (NEW): watches the active-proxy stream and deactivates a profile after 3 consecutive url-test rounds where ALL its outbounds are unreachable (delay == 0 or >= 65000). The connection notifier then reconnects with the remaining (healthy) profiles — i.e. the app 'switches to a different profile by itself'.
• Wired AutoProfileSwitchNotifier into ConnectionWrapper.

NOTE: run 'dart run build_runner build --delete-conflicting-outputs' to
regenerate .g.dart files for the new/changed Riverpod notifiers
(ActiveProfiles, AutoProfileSwitchNotifier, mergedConfigBuilderProvider).