Rewrite CLI in Rust (clap + ratatui), move releases to cargo-dist

[nklmilojevic]

Summary

• Full rewrite of the Go CLI in Rust: all 22 command groups (subscriber, group, campaign, automation, form, field, segment, webhook, timezone, 8 e-commerce groups, account, auth, profile, completion, version) with identical subcommands, flags, defaults, output columns, and messages
• ratatui dashboard replacing bubbletea: sidebar, five views, detail/help overlays, vim keybindings, non-blocking background fetches, panic-safe terminal restore
• Full OAuth PKCE browser flow (localhost:19821 callback), multi-profile YAML config with auto token refresh, retry/rate-limit HTTP client with verbose logging
• Releases move from GoReleaser to cargo-dist: native per-platform builds, shell/PowerShell installers, Homebrew formula (replaces cask) published to mailerlite/homebrew-tap, flake.nix hash update as post-announce job
• Version is now authoritative in Cargo.toml (tag must match); build.rs stamps git commit/date
• CI quality gates (fmt, clippy -D warnings, build, test) now run on pushes to main and PRs

Breaking / operational changes

• Homebrew: cask -> formula; existing cask users need brew uninstall --cask mailerlite && brew install mailerlite/tap/mailerlite (cask already removed from tap)
• macOS binaries are no longer signed/notarized (cargo-dist has no macOS signing); QUILL_* secrets unused
• nix run and brew install are broken until the first v2.0.0 tag publishes artifacts with the new names
• --json prints the raw API response body (higher fidelity than the Go SDK-struct re-marshal)

Test plan

• cargo build + clippy --all-targets: zero warnings
• Automated parity diff vs the Go binary: all 20 command groups and every leaf subcommand's flags match exactly
• Live API smoke: auth status, subscriber count/list, group list --json, --verbose logging, group create/delete round-trip
• Dashboard driven in a real PTY: load, view switch, help overlay, clean exit
• dist plan + local dist build: tarball layout matches flake sourceRoot; extracted binary works against live API
• Workflows pass action-validator; flake evaluates
• First v2.0.0 tag: verify release artifacts, tap formula, and update-flake bot commit