Restyle the UI to match lumeex's back-office look, add repo links

Reworked static/style.css to follow the same visual language as
lumeex's back office (near-black background, cyan accent, gradient
pill buttons, translucent bordered panels) instead of the previous
generic dark-teal theme. Dropzones now use a dashed border while
empty, turning solid once a file is picked. Also fixes a stale CSS
selector (.dropzone label.tag) that never matched the actual markup
(a <span>), so the SDR/HDR tag labels were unstyled before.

Adds Gitea and GitHub repo links in the footer, using the same icons
and link pattern as lumeex.

README.md

@@ -13,15 +13,8 @@ itself based on the reverse-engineering work of
 [karachungen/instagram-hdr-converter](https://github.com/karachungen/instagram-hdr-converter).
 Original MIT license kept in [LICENSE](LICENSE). 
 
-Only the upstream's Method 2 (starting from an HDR JPEG that already
-carries a gain map) is implemented here. Method 1's AVIF decoding and
-its quality/transfer/gamut settings were dropped, but the HDR side can
-still be a 32-bit float linear TIFF (Lightroom's HDR TIFF export)
-instead of a gain-map JPEG for a more precise result. Either way the
-gain map is recomputed rather than reused as-is (see below), since it
-needs to be correct against the SDR file you actually provide.
 
-![Instagram HDR Assembler screenshot](screenshots/insta-hdr.png)
+![Instagram HDR Assembler screenshot](screenshots/insta-hdr-1.png)
 
 ## How it works