The archive view's `fuzzy_match_positions` used `chars().enumerate()`
which produces **character indices**, not **byte indices**. When thread
titles contain multi-byte UTF-8 characters (emoji, CJK, accented
characters, etc.), these character indices don't correspond to valid
byte boundaries, causing a panic in `HighlightedLabel::new` which
asserts that highlight indices are valid UTF-8 boundaries.

The fix switches to `char_indices()` and `eq_ignore_ascii_case()` to
produce correct byte positions, matching the approach already used by
the sidebar's version of the same function.

Release Notes:

- Fixed a crash when searching archived threads whose titles contain
emoji or other non-ASCII characters.