chore: add FSL-1.1-MIT licence and fill in package metadata

The repo had no licence at all, which legally means all rights reserved. FSL grants free use for
anything except building a competing product, and each version becomes MIT two years after its
release. AGPL was the other candidate and is ruled out by the App Store, whose terms impose
restrictions the GPL forbids.

Cargo.toml still carried the Tauri scaffold defaults, and that description ends up in the deb and
rpm metadata.

Claude-Session: https://claude.ai/code/session_018HwEWvoE1NkkUjJCMLSnup

src-tauri/Cargo.toml

@@ -1,8 +1,9 @@
 [package]
 name = "margin-app"
 version = "0.1.17"
-description = "A Tauri App"
-authors = ["you"]
+description = "A calm, offline writing studio for authors"
+authors = ["Priyanshu Jain"]
+license = "LicenseRef-FSL-1.1-MIT"
 edition = "2021"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html