feat(frontend): add native image viewer

[sandersonstabo]

Adds a native GPUI image viewer with controlled lifecycle state, theme-aware modal presentation, deterministic geometry, dismissal semantics, and focused behavior/Clippy coverage.

Stacked on #165.

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Summary by cubic

Adds a native gpui full-screen image viewer with pure controlled state, theme-aware styling, and deterministic fitting geometry. The viewer never decodes images and never manages inspection resources directly; it only queues retain/release actions for the caller to drain and execute.

Design notes

• Open/close state, geometry, and style are pure types tested without a GPUU window.
• gpui 0.2.2 lacks backdrop filters, z-index, and an OS accessibility tree, so those are kept as typed intents and sibling paint order.
• Backdrop click, close button, and Escape all close through the same controlled transition and restore the previous focus.

^{Written for commit 5ca2c1c. Summary will update on new commits.}