Standardize entry titles (HTML, case, whitespace) before checking for duplicates in feeds and categories.
Without these changes, some users reported "hit or miss" behaviour after enabling Mark an article as read… if an identical title already exists in the top n newest articles of the category (#948 (comment), #948 (comment), #948 (comment)).

Closes #

Changes proposed in this pull request:

• Added FreshRSS_Entry::normalizeTitle() to decode HTML entities, collapse whitespace, and lowercase strings.
• Applied this normalization to the $titlesAsRead lookup in both Feed and Category scopes.

How to test the feature manually:

1. Subscription management > Add a feed or category > enter "RNZ" under Add a category, click Add

2. Click the gear icon ⚙️ on the left of the RNZ category, under Filter actions tick Mark an article as read... ☑️ if an identical title already exists in the top n newest articles of the category, enter 100, click Submit

3. Back to RNZ category, click ➕ Add a feed

then paste URL of "RNZ World Headlines" feed. Use default settings for Article unicity criteria.
Repeat for "RNZ New Zealand Headlines".

4. Back to main page of FreshRSS, click on the Show read button (opened envelope icon) in the nav menu, I can see 1 out of 2 entries of the article "Iranian diaspora form human chain on Wellington waterfront" is correctly marked as as read.

5. Need to observe for some time to make sure no more "hit or miss" behaviour.

Pull request checklist:

• clear commit messages
• code manually tested
• unit tests written (optional if too hard)
• documentation updated

Additional information can be found in the documentation.