feat(search): shared "search in page" across file viewer, diff viewer & content-search preview

[matej21]

Context

Okena renders file/text content line-by-line in three places, but only one let you find text inside it:

View	Renders content	In-page find (Cmd/Ctrl+F) before
File viewer	yes	yes
Git diff viewer (unified + side-by-side)	yes	no
Content Search dialog preview pane	yes	no

The file viewer's find lived entirely inside impl FileViewer (~340 lines, ~130 of which was the search-bar UI). Copying it twice would triple the bug surface and the UI drift — already visible in an inconsistency: the file viewer hardcoded its highlight colors while the content-search preview already read theme colors.

What this does

Extracts the find logic + search-bar UI into one shared, view-agnostic module and drives all three viewers from it.

Shared module — crates/okena-files/src/in_page_search.rs (new)

• InPageSearch engine: searches a flat list of host-defined cells (text + opaque usize id), tracks the current match, navigation, case toggle, and theme-colored ranges_for_cell(). The host owns all view semantics (what a cell maps to, how to scroll to it).
• Generic render_search_bar<V> + SearchBarCallbacks<V>: the full bar (input, Aa case toggle, n/N counter, prev/next, close). Handles Enter / Shift+Enter and stops key propagation so typed keys never leak into host shortcuts; Escape flows to each host's existing root Cancel handler (which closes search first).

Hosts

• File viewer — refactored onto the shared engine. Behavior-preserving; also unifies the previously hardcoded highlight colors onto theme colors.
• Git diff viewer — in-page search added for both unified and side-by-side layouts. Cell scheme: unified = item index (expanders get an empty cell so indices line up with the uniform_list); side-by-side = row*2 + side, scroll item = cell / 2. Matches recompute on file switch / view-mode toggle / whitespace toggle / query / case change via a signature check.
• Content Search dialog — in-page search over the preview pane (Cmd/Ctrl+F), layered on top of the existing grep-match highlights.

Verification

• cargo build --workspace — OK (incl. the okena binary and okena-app)
• cargo test --workspace — all pass, 0 failures
• cargo clippy -p okena-files -p okena-views-git — clean

Manual GUI verification of the three views (interactive find/highlight/scroll, both diff layouts) still recommended.

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