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/android/src/main/res/values/strings.xml

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  App name as shown on the home screen, in the app drawer and in the task
+  switcher.
+
+  `tauri android init` generates this file from `productName`, and its output
+  was the lowercase "jellytau" that shipped in every release build. The mistake
+  was invisible during development because build.gradle.kts overrides
+  manifestPlaceholders["appLabel"] to "JellyTau Debug" for the debug build type,
+  so the side-by-side install a developer looks at every day was correctly
+  cased — only the release users install was wrong.
+
+  Held in the canonical android/src tree so sync-android-sources.sh copies it
+  over the generated one (it already syncs res/values/*.xml for themes.xml),
+  which keeps it from being lost the next time gen/ is regenerated.
+
+  TRACES: | DR-214
+-->
+<resources>
+    <string name="app_name">JellyTau</string>
+    <string name="main_activity_title">JellyTau</string>
+</resources>