Add classic and spacious TUI layouts

[TyRichards]

Summary

• keep the current edge-to-edge TUI as the default classic layout
• add hey tui spacious with vertical breathing room around content, between mail rows, and around section headers
• streamline the spacious footer while preserving the full terminal width
• let readers switch live with Ctrl+G and collapse the added spacing automatically on narrow terminals
• reject layout arguments for --remote, which opens an already-running TUI

Demo

Testing

• make fmt-check vet tidy-check check-surface check-release-lockstep
• golangci-lint run ./...
• make test

[Copilot]

Pull request overview

Adds selectable classic and spacious TUI layouts with runtime switching and responsive spacing.

Changes:

• Adds layout selection and Ctrl+G toggling.
• Introduces spacious content, row, header, and footer spacing.
• Documents and tests layout behavior.

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Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
README.md	Documents layout selection and switching.
internal/cmd/tui.go	Parses and validates layout arguments.
internal/cmd/root_test.go	Tests CLI layout selection.
internal/tui/layout.go	Defines layouts and responsive metrics.
internal/tui/layout_test.go	Tests layout rendering and toggling.
internal/tui/tui.go	Integrates layout sizing and switching.
internal/tui/tui_test.go	Adds Ctrl+G test input support.
internal/tui/content.go	Adds spacing between mail rows and sections.
internal/tui/mail.go	Applies layout gaps to mail lists.
internal/tui/toast.go	Sizes toasts against content width.
internal/tui/section_view.go	Shares layout metrics with views.
internal/tui/accounts.go	Sizes account modals to content width.
internal/tui/accounts_test.go	Updates model construction calls.

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[jeremy]

Rebased onto main at 6cf8eba (the README paragraph moved to docs/tui.md, where the manual now lives) and make check is green; all three review threads are fixed on the branch.

Where this stands: whether HEY's TUI takes on a second layout is a product call that is still pending, so the PR is parked on that decision rather than on anything in the diff. It is current and green in the meantime.