Rename the project to Instameex

Renames the app throughout: page titles, eyebrow/version labels,
Docker startup banner, main.py's header docstring, and the repo
links in the footer (now pointing at git.djeex.fr/Djeex/instameex
and github.com/Djeex/instameex, matching the renamed Gitea/GitHub
repos). Docker image and container are now named "instameex" too
(docker/compose.yaml), replacing the old instagram-hdr naming.

Adds the new logo: logo.svg is wired up as the favicon on both
pages, and logo-long.svg replaces the plain-text "Instameex" eyebrow
on the upload page as a properly sized wordmark.

The startup banner box width was recalculated by hand for the new
content (name plus Source:/Mirror: links instead of the old
license/upstream-credit lines), verified against a real container
rebuild so the border characters stay aligned.

kostis-kounadis/instagram-hdr-assembler references are left alone
throughout (README and banner): that's the separate upstream project
this app's gain-map logic is adapted from, not this app's own name.

docker/compose.yaml

@@ -1,11 +1,12 @@
-name: instagram-hdr
+name: instameex
 
 services:
   hdr-web:
     build:
       context: ..
       dockerfile: docker/Dockerfile
-    container_name: instagram-hdr-web
+    image: instameex
+    container_name: instameex
     restart: unless-stopped
     ports:
       - "5050:5000"  # host:container — change 5050 if that's taken too