feat: plugin marketplace with in-app browsing and install

[datlechin]

Summary

• Add Browse tab to Settings > Plugins with search, category filtering, and one-click install from the TableProApp/plugins registry
• Registry client fetches flat JSON manifest with ETag caching and offline fallback
• Install flow: streaming download with progress → SHA-256 checksum verification → delegates to existing installPlugin(from:)
• Extract existing installed plugins UI into InstalledPluginsView, restructure PluginsSettingsView as Installed/Browse sub-tab wrapper

New files (backend)

• Core/Plugins/Registry/RegistryModels.swift — RegistryManifest, RegistryPlugin, RegistryAuthor, RegistryCategory
• Core/Plugins/Registry/RegistryClient.swift — fetch, ETag cache, search, offline fallback
• Core/Plugins/Registry/PluginInstallTracker.swift — per-plugin download/install state
• Core/Plugins/Registry/PluginManager+Registry.swift — installFromRegistry() with checksum verification

New files (UI)

• Views/Settings/Plugins/InstalledPluginsView.swift — extracted from PluginsSettingsView
• Views/Settings/Plugins/BrowsePluginsView.swift — search bar, filter chips, plugin list
• Views/Settings/Plugins/RegistryPluginRow.swift — plugin card with verified badge
• Views/Settings/Plugins/RegistryPluginDetailView.swift — expandable detail panel

Test plan

• Build succeeds with xcodebuild -skipPackagePluginValidation
• Settings > Plugins shows Installed/Browse segmented picker
• Installed tab unchanged from Phase 2
• Browse tab fetches registry, shows loading then empty state (registry has no plugins yet)
• Search and category filter chips work
• Error state shown on network failure, "Try Again" button refetches
• Existing test suite passes (no regressions)

[chatgpt-codex-connector]

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