Replace the search indicator with a real editable field

[alvst]

The search UI was a synthetic pill assembled from captured keystrokes: append-only, with no cursor, no click-to-edit, no selection, and no IME. This replaces it with a real NSTextField edited through the normal AppKit responder chain.

What changed

• NativeBarSearchField is a real text field: cursor, click-to-edit, text selection, ⌘A/⌘X/⌘C/⌘V, and IME/dead-key input all work.
• The field stays mounted even in its compact zero-width state, so the bar's key monitor can focus it synchronously and the very first typed character isn't lost.
• BarInputMode tracks whether the field or the clip strip owns focus, so the key monitor and the field agree on who handles a given key instead of probing the responder chain from every call site.

Screenshots

Before:

After:

Notes for review

• The first-responder guard that keeps bar shortcuts alive while typing is intact — verified that ⌘-shortcuts still reach the bar with the field focused.
• Related: alvie/pr-27-space-preview-with-search exists because this branch and alvie/pr-quicklook interact — a focused field would otherwise swallow Space-to-preview. That's fixed in its own PR, not here; nothing in this branch regresses on its own.
• Out of scope: no search scoping, history, or regex.

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This feature should be bundled into v2.

Part of #80.