feat(config): named Typesense clusters with picker

[tharropoulos]

• add [clusters.<name>] plus global.default_cluster. legacy flat url/api_key migrates to a default cluster
• expose Config.URL() / APIKey() / ActiveCluster() / ClusterNames() helpers and update the app to route through them
• gain a Cluster field in the Setup form
• in Settings, cycle through configured clusters (and a trailing + new slot) with ← / → on the Cluster field; URL and API key auto-replace from the selected cluster
• open a global cluster picker with c from any normal tab. Enter saves global.default_cluster, rebuilds the client/screens, and refreshes the current tab
• render the global picker as a centered overlay on top of the current page (new internal/tui/overlay.go)
• support j/k aliases alongside ↑/↓ in the global picker
• document the named-clusters config layout and picker keys in the README

depends on: #4

[Copilot]

Pull request overview

This PR introduces named Typesense clusters backed by a TOML config format, and adds TUI affordances to switch clusters both from Settings and via a global cluster picker overlay, routing all connection usage through new config.Config helper methods.

Changes:

• Replace legacy flat YAML connection config with TOML [clusters.<name>] plus global.default_cluster, including legacy YAML fallback/migration and tab-order validation.
• Add a global cluster picker (c) rendered as a centered overlay, plus Settings/setup support for selecting/creating clusters and auto-filling URL/API key.
• Update the TUI app wiring to use configurable tab ordering and Config.URL()/APIKey()/ActiveCluster... helpers, with new tests covering these behaviors.

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
README.md	Documents TOML config layout, named clusters, and picker/navigation keys.
main.go	Uses cfg.NeedsSetup() to decide whether to enter setup even when a config file loads.
internal/tui/screens/setup.go	Adds Cluster field and cluster cycling/new-cluster slot behavior in the setup form.
internal/tui/overlay.go	Introduces overlay composition helpers for rendering a centered modal over existing content.
internal/tui/app.go	Adds global cluster picker state/handlers, tab ID/title configuration, and routes client creation through config helpers.
internal/tui/app_test.go	Adds tests for tab ordering, settings save behavior, cluster switching, overlay rendering, and Ctrl+C behavior in setup.
internal/config/config.go	Implements TOML config (plus legacy YAML fallback), cluster selection helpers, tab-order validation, and migration utilities.
internal/config/config_test.go	Adds/updates tests for TOML round-trips, legacy YAML fallback, NeedsSetup, tab order, and invalid tabs errors.
go.mod	Adds go-toml/v2 dependency.
go.sum	Adds go-toml/v2 checksums.

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