feat(dictionary): add Hachidori backend support

- Add backend selection, setup gating, Anki integration, and external host support
- Add launcher flags, documentation, packaging, and focused tests
- Open on-demand overlay modals on the first attempt

vendor/hachidori/extension/assets/ATTRIBUTION.md

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+# Visual novel backgrounds
+
+Copyright © 2026 bee-san.
+
+`preview-background.webp` is the repository owner's supplied artwork from
+`ChatGPT Image Sep 8, 2026, 06_29_20 AM.png`. On September 8, 2026, the owner
+confirmed that they own its copyright and requested its use in Hachidori, with
+ownership documented for Chrome publishing.
+
+`preview-background-2.webp` through `preview-background-6.webp` are additional
+owner-supplied scenes, from the September 8, 2026 images made at 07:16:27,
+07:16:31, 07:16:38, 07:16:43 and 07:18:10, respectively. The owner requested
+their inclusion in the first-run and Design scene rotation.
+
+Each packaged file is that artwork cropped to the 1672 × 672 panel the scene
+displays, encoded as WebP.
+
+See the [asset ownership and publishing record](../../docs/asset-rights.md) for
+the source identities, checksums, logo inventory and authorized project use.