feat: default search to case-insensitive

[jelloeater-agent]

Summary

Change the default of the buffer/editor search to be case-insensitive (like micro). New searches now match case-insensitively without any toggling.

What changed

• crates/fresh-editor/src/app/window/mod.rs: flip the initial search_case_sensitive from true to false. This runtime field drives both the search toggle and new searches.

Behavior intentionally unchanged:

• The per-search toggle_search_case_sensitive action and the [x] Case Sensitive status-bar checkbox work exactly as before (default is now [ ], toggling flips it).
• Serialized workspace state: SearchOptions already serde-defaults to false; saved states that explicitly stored case_sensitive are still restored as saved.
• No smart-case behavior, no new config keys.

Why just this one field

SearchOptions::default() already yields case_sensitive: false via #[serde(default)], but the runtime search_case_sensitive field on Window was initialized to true, which drove every new search. Flipping the init is the minimal, root-cause change.

Tests

• Updated test_toggle_case_sensitive_in_search (e2e) to assert the checkbox is now unchecked ([ ]) by default and checked ([x]) after Alt+C.
• Verified: cargo check -p fresh-editor compiles; cargo fmt --check clean; e2e e2e::search:: 48/48 pass, search_replace 50/50 pass (2 ignored, pre-existing); workspace serialization unit tests (test_search_options_serialization, test_full_workspace_round_trip, test_workspace_file_save_load) pass.