feat: Add watchlist UI

[SloppyTurtle]

Overview

Add the frontend UI for managing the author/series watchlist.
Depends on #1 (watchlist database schema and backend API).

Goal

Give users a clean interface to search for authors, add them to their
watchlist, view detected releases, and manage watchlist entries.

Pages/Components to Add

Watchlist Page (/watchlist)

• List of watched authors with author name, provider, last checked timestamp, release count, enable/disable toggle, remove button
• Empty state when no authors watched
• Add Author button opens search modal

Add Author Modal

• Search input querying Hardcover/Open Library metadata providers
• Results show author name, photo if available, book count
• Add button per result
• Duplicate prevention (grey out already-watched authors)

Releases View

• Accessible from each author row
• List of detected releases with title, format, release date, status badge
• Queue and Ignore buttons per release
• Filter by status

Navigation

• Add Watchlist link to existing sidebar
• Follow existing sidebar pattern

Acceptance Criteria

• Watchlist page renders with and without entries
• Add Author modal searches and displays results
• Authors can be added and removed
• Authors can be enabled/disabled
• Releases can be queued or ignored
• Follows existing Shelfmark component patterns
• Responsive layout consistent with rest of app
• Loading and error states handled
• No changes to existing pages or components

Out of Scope

• Scheduler/background jobs (issue feat: Add watchlist UI #3)
• Auto-queue logic (issue feat: Auto-queue new releases into download queue #4)

Depends On

• feat: Add watchlist database schema and backend API #1 watchlist database schema and backend API