docs(user): dashboard, browsing, profile rewrites + new podcasts guide; trash stubs

Ground-truthed against the templates this pass:
- dashboard.md: real section kinds (system shelves incl. Continue
  Series, user collections, custom sections via /custom-section
  builder), Customize Dashboard panel semantics (per-library, drag,
  switches, 10-50 items, Save Changes), Refresh, This Week widget,
  per-item Read/Listen and progress-conflict resolve actions. The old
  page listed a wrong shelf set and never mentioned custom sections.
- library-browsing.md: the file was secretly titled 'Saving Custom
  Filters' and covered nothing else. Now the browsing page: All Books
  toolbar (search, Sort, Filters -> drawer hand-off, Save/Load/Clear
  saved filters), book detail as hub, series pages, podcasts split.
- profile-guide.md: field table ground-truthed from profile_form.templ
  (username/email DO remain editable there), audio defaults (speed,
  resume rewind, keep awake), timezone rendering contract, both theme
  doors (7-pick select vs 37-theme Appearance panel), Change Password
  with live checks, Danger Zone/Remove My Account + last-admin rule.
- podcasts.md NEW: the Podcasts page had zero documentation. Subscribing
  (Add Feed -> RSS URL, library, back-catalog download, download order,
  Add & Subscribe), Continue/All Feeds, Download All, player + Play as
  Sequence cross-link.
- Trashed user-areas.md, frontend-guide.md, settings-guide.md: stubs
  whose planned content now exists as real pages (or duplicates
  profile-guide). user-guide.md rewritten as the plain-language front
  door with a go-here-when table; index.md rows updated to match.
- ui-controls.md: 10 new gate-checked entries for every newly cited
  control (Add & Subscribe via templ-entity fragment).

docs/index.md

@@ -27,13 +27,12 @@ Complete guide to Bookhoard documentation. Find what you need quickly.
   - [Reader Guide](user/reader-guide.md) - reading, highlights, notes
   - [Library Browsing](user/library-browsing.md) - browsing and searching your library
   - [Collections](user/collections.md) - manual and ⚡ smart shelves, books and episodes
+  - [Podcasts](user/podcasts.md) - feeds, downloads, listening
   - [Dashboard](user/dashboard.md)
   - [Themes](user/themes.md)
   - [Calibre Integration](user/calibre-integration.md)
-  - [Settings Guide](user/settings-guide.md)
   - [Profile Guide](user/profile-guide.md)
   - [Admin Guide](user/admin-guide.md)
-  - [Frontend Guide](user/frontend-guide.md)
 
 ### 👨‍💻 For Developers