Docs: add Getting started, Mobile, and FAQ pages #352

Problem. The docs have no first-run or mobile walkthrough (docs/docs/ has api, commands, import_export, index, local_files, podcasts, QuickAdd, templates, timestamps, transcripts). Recurring questions go unanswered: Android guide, whether downloads sync and what happens to links, Spotify exclusives, local video annotation. docs/docs/*.md also contains 12 em-dash characters (grep), which the program style forbids.

Acceptance. New pages exist with screenshots from the redesigned UI, and npm run docs:build passes.

Found by the PodNotes user-feedback research (Reddit, Obsidian forum, YouTube, GitHub history).