feat: add option to hide duplicate articles within the same feed

[nethbotheju]

Problem

Some feeds publish the same article multiple times (same title, slightly different
timestamp/link), which clutters the article list, the unread view, and the unread
badge counts.

Changes

Adds a new setting Settings → Usage → Article list → "Hide duplicate articles"
(default off). When enabled, an article is hidden if an earlier article with the
same title exists in the same feed — the oldest copy is always the one kept.

Applied consistently to:

• Article lists: feed view, group view, and the account-wide flow (Unread + All)
• In-app search results (Unread + All)
• Unread/All badge counts on the Feeds page, so the numbers match the visible list

Design decisions

• Scoped per feed: identical titles from different feeds are both kept —
  two outlets reporting the same story is not a duplicate.
• Keep the oldest: strictly the earliest article wins, so marking the visible
  copy as read does not resurface a newer unread duplicate in the Unread view.
• SQL-level filtering (NOT EXISTS subquery in the DAO queries) rather than
  Kotlin-side filtering, so deduplication stays correct across Paging page
  boundaries, and the same clause keeps badge counts consistent.
• Starred lists are untouched — those are explicitly curated by the user.
• Exact title match; empty titles never deduplicate; ties on date break by article id.
• Display-only: nothing is deleted and there is no schema change/migration;
  toggling off restores the full list immediately.
• The pager and badge jobs react to the toggle without needing to re-enter the screen.

Testing

• New unit tests for the preference (default off, stored value, fallback).
• Full existing unit test suite passes (testGithubDebugUnitTest).
• Built the github release APK from CI and tested on a physical device:
  duplicates collapse to the oldest entry in feed/group/unread views, badge counts
  match, cross-feed same-title articles both remain, starred view unaffected,
  toggling off restores everything.

Screenshots

Toggle in Settings	Before	After