Fix hashtag filter inconsistency causing notes not to display in channels

This commit fixes a critical bug where hashtag filters were created
inconsistently, preventing notes from displaying in channel timelines.

Problem:
- TimelineKind::filters() was creating subscription filters with
  lowercase hashtags: .tags([tag.to_lowercase().as_str()], 't')
- But Timeline::hashtag() and TimelineKind::into_timeline() for Relay
  variant were creating query filters WITHOUT lowercasing
- This caused a mismatch when querying nostrdb, as Nostr hashtags
  are case-insensitive and stored lowercase
- Result: Notes were received (66 notes in bitcoin channel) but not
  displayed in the GUI

Solution:
- Make ALL hashtag filter creation use .to_lowercase() for consistency
- Updated Timeline::hashtag() (mod.rs:279)
- Updated TimelineKind::into_timeline() Relay variant (kind.rs:668)

This ensures subscription filters and query filters use the same
lowercase hashtag format, allowing notes to properly display in channels.

crates/notedeck_columns/src/timeline/kind.rs

@@ -665,7 +665,7 @@ impl TimelineKind {
                             Filter::new()
                                 .kinds([1])
                                 .limit(filter::default_limit())
-                                .tags([tag.as_str()], 't')
+                                .tags([tag.to_lowercase().as_str()], 't')
                                 .build()
                         })
                         .collect::<Vec<_>>()

crates/notedeck_columns/src/timeline/mod.rs

@@ -276,7 +276,7 @@ impl Timeline {
                 Filter::new()
                     .kinds([1])
                     .limit(filter::default_limit())
-                    .tags([tag.as_str()], 't')
+                    .tags([tag.to_lowercase().as_str()], 't')
                     .build()
             })
             .collect::<Vec<_>>();