feat: saved request/response snapshots (replay + diff, git-synced)

[axbarchuk]

What & why

Adds saved snapshots — immutable, git-synced captures of a fully-resolved request and its response, so anyone (or an AI agent over MCP) can later check whether an API still behaves the way it did when captured.

• Save an HTTP response + its as-sent request as an immutable snapshot_*.yaml in the workspace dir. Secrets are encrypted at rest or redacted — never plaintext.
• Snapshots are expandable children of their request in the sidebar (pin, rename, multiselect, cascade-delete); search and the git Changes window include them.
• Read-only view reuses the live request/response components.
• Replay re-runs the request; a Snapshot / Replay / Diff switch compares saved vs latest (git-style unified diff of body/headers/cookies).
• MCP tools: snapshot.save/list/get/replay (masked by default).

Also includes UX polish built alongside: collapsible panes (drag + ⌥⌘←/→), tinted pane top-chrome, responsive one-line headers, smaller min window, and tree keyboard/selection fixes.

Checklist

• bun run typecheck, bun run lint, and the cargo checks pass
• New Tauri commands registered in collect_commands! and bun run codegen run
• Follows AGENTS.md (file-size limits, render-loop rules, no migration shims)