docs: add safe setup troubleshooting guide

## Goal Create a concise troubleshooting guide for common setup failures that occur before any media handling begins. ## Scope Add a documentation page or clearly linked section covering safe, local setup problems such as: Go not found or too old; the CLI binary missing from `PATH`; optional FFmpeg not found; an unpacked extension manifest error; and a Native Messaging host executable or allowlist mismatch. Each entry should explain a diagnostic command, a minimal expected result, and a safe next step. ## Acceptance criteria - Keep the guide aligned with the root Go module and `cli/` package layout. - Use sanitized command output examples only. - Explain that FFmpeg is optional for direct files and required only for applicable local HLS/DASH remuxing. - Link to the installation verification checklist and Native Messaging host guide where useful. - Remind users not to post private URLs, cookies, headers, credentials, tokens, DRM keys, or protected media in support requests. ## Out of scope Do not add login support, cookie import, custom authorization headers, DRM handling, access-control bypasses, proxy workarounds, or media acquisition tests.