chore: give the project its own identity instead of the scaffold's

Cargo.toml still carried `description = "A Tauri App"` and `authors = ["you"]`,
package.json's description was empty with no author or repository, and there was
no LICENSE file at all despite package.json declaring MIT.

The user-visible half matters more. productName was the scaffold's lowercase
"jellytau", which is what the Android *release* build shows under its icon and
what the deb/rpm/NSIS bundles carry as their display name. It went unnoticed
because build.gradle.kts overrides the label to "JellyTau Debug" for the debug
build type — the install a developer sees every day was the only correctly-cased
one. mainBinaryName pins the executable filename to "jellytau" so
build-windows-cross.sh and the Arch PKGBUILD, which both resolve it by name,
need no change.

strings.xml moves into the canonical android tree rather than being edited in
gen/, since sync-android-sources.sh already copies res/values/*.xml — so the fix
survives the next regeneration.

Bundle metadata (publisher, copyright, category, descriptions, licence) was
absent entirely, so the packages shipped with no maintainer or description. The
hand-written PKGBUILD and .desktop had all of it; only the generated packaging
was wrong.

Adds .env.example: three scripts require signing vars from a gitignored .env
and .gitignore already whitelists the example, but none existed.

TRACES: | DR-214

src-tauri/Cargo.toml

@@ -1,8 +1,10 @@
 [package]
 name = "jellytau"
 version = "0.8.2"
-description = "A Tauri App"
-authors = ["you"]
+description = "A cross-platform Jellyfin client"
+authors = ["Duncan Tourolle <duncan@tourolle.paris>"]
+license = "MIT"
+repository = "https://gitea.tourolle.paris/dtourolle/jellytau"
 edition = "2021"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html