Add Docker Hub image search (Explore)

[kylemclaren]

Summary

A new Explore screen that searches Docker Hub and pulls straight into the local container store — the one competitive gap versus comparable tools.

• New network seam (Core/Net/HTTPClient.swift): HTTPClient protocol + URLSessionHTTPClient, the network-side counterpart to CommandRunner. The container CLI has no registry-search command, so this is the one place the app talks to the network directly; it's mockable for tests.
• DockerHubService over Hub's legacy /v2/search/repositories/ (flat schema, exact star/pull counts) plus a per-repo tags endpoint. Models decode Hub's snake_case and nanosecond timestamps (dates kept as String).
• Explore feature: debounced, cancellable search; Official badges + star/pull counts; a per-repo tag inspector (size / platform / date); one-click Pull that pre-fills the existing pull sheet with docker.io/<ns>/<repo>:<tag>.
• Wiring: a distinct accent Explore button pinned at the bottom of the sidebar, a "Search Docker Hub…" command-palette action, and dockerHubService on AppModel.

Tests

New DockerHubTests suite: URL building (percent-encoding), search/tags decoding, HTTP + decode error mapping, compactCount (including unit-boundary regressions), and ISO8601 parsing. Full suite: 106 tests, 14 suites, all passing; app target builds clean.

Notes

• Adversarially reviewed; three confirmed issues fixed (results/inspector preserved on transient refine-errors, compactCount unit-boundary promotion, tag-flash on repo switch).
• Also folds in the previously-added README Homebrew install section.

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