feat(booth): image viewer page with Fit/1:1, download, Esc-back (v0.1.2)

Clicking a gallery image now opens a dedicated viewer instead of dumping you on
the raw file.

- GET /b/<name>/view?f=<img> — full-viewport viewer (registered before the file
  catch-all so /view wins; non-image f 307-redirects to the raw file, traversal
  and missing f 404).
- Fit (downscale-only) / 1:1 (natural pixels, scroll-to-pan) toggle that only
  appears when the image is larger than the viewport — when it already fits,
  Fit ≡ 1:1 so the toggle is hidden. Re-evaluates on resize.
- Download button + ✕/Esc back to the gallery. Australis-themed, progressive
  JS (degrades to fit-only, no-JS still shows the image + download + back).
- 5 new tests (34 total, all green); verified Fit/1:1/hidden-toggle states in a
  real browser.

README.md

@@ -66,7 +66,9 @@ to a safe basename (no path traversal).
   `chart.png`, `report.css` — resolve out of the same folder). Build whatever
   page you want.
 - **No `index.html`?** → **auto-gallery** of the folder's media:
-  - images (`png jpg jpeg gif webp avif svg bmp`) → `<img>`
+  - images (`png jpg jpeg gif webp avif svg bmp`) → `<img>` (click → full-screen
+    viewer with **Fit** / **1:1** — the toggle only appears when the image is
+    larger than the viewport — plus download and ✕/Esc back to the gallery)
   - video (`webm mp4 ogv m4v mov`) → `<video controls>`
   - audio (`mp3 wav ogg flac m4a opus aac`) → `<audio controls>`
   - anything else → a download link