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

package.json

@@ -1,7 +1,14 @@
 {
   "name": "jellytau",
   "version": "0.8.2",
-  "description": "",
+  "description": "A cross-platform Jellyfin client built with Tauri, SvelteKit and Rust.",
+  "author": "Duncan Tourolle <duncan@tourolle.paris>",
+  "license": "MIT",
+  "repository": {
+    "type": "git",
+    "url": "https://gitea.tourolle.paris/dtourolle/jellytau"
+  },
+  "private": true,
   "type": "module",
   "packageManager": "bun@1.3.5",
   "scripts": {
@@ -47,7 +54,6 @@
     "traces:validate": "bun run scripts/extract-traces.ts --format validate",
     "release:notes": "bun run scripts/release-notes.ts"
   },
-  "license": "MIT",
   "dependencies": {
     "@tauri-apps/api": "^2",
     "@tauri-apps/plugin-opener": "^2",