fix: ship the licences of the Rust libraries inside MemoryStick

[MotherSphere]

MIT, BSD, Apache-2.0, ISC and similar licences require a binary distribution to carry their notices. The executable statically links about 320 Rust crates, and until now only Cargo.lock was pointed at.

What changes

• src-tauri/build.rs runs $CARGO metadata --format-version 1 --locked --offline --filter-platform $TARGET, follows the normal dependencies reachable from memorystick (build and dev dependencies are not linked in), and reads each crate's LICENSE*, LICENCE*, UNLICENSE*, COPYING*, COPYRIGHT* and NOTICE* files plus any declared license-file. Texts that differ only in whitespace are kept once, under the sorted name@version (SPDX) list of crates that ship them. Crates with no licence file are listed at the end with their SPDX expression. The result goes to $OUT_DIR/rust-licenses.txt. No new dependency: serde_json was already in the lockfile.
• src/licenses.rs embeds that file and exposes it through a new third_party_licenses command, declared in build.rs and allowed in capabilities/default.json (allow-third-party-licenses).
• Preferences > About gains a collapsible "Third-party licences" section. The text is fetched the first time it opens and shown in a scrollable, focusable pre. New strings preferences_third_party and preferences_third_party_desc in English and French (the existing parity test covers them).
• docs/third-party-notices.md replaces the Cargo.lock sentence with how the notices travel in the binary; the README licence section mentions them.

Offline behaviour

--offline means the build script never reaches the network: it only reads packages Cargo already downloaded to build them. With --filter-platform, cargo metadata needs exactly the target and host packages the build itself fetched. Verified locally on Linux (no lock contention with the outer build); this PR's CI exercises Windows, macOS and the MSRV toolchain.

Binary size

The embedded text for x86_64-unknown-linux-gnu is 536,217 bytes (168 distinct licence texts for 322 crates; whitespace-only duplicates of the Apache-2.0 text are merged). It is stored uncompressed, so the release binary grows by about 536 KB, about 4.5% of v0.1.1's memorystick-linux (11,899,808 bytes). Windows and macOS link a different set of crates, so their text and delta differ slightly.

Tests

• licenses::tests::names_the_libraries_memorystick_is_built_on: the generated text is non-empty and lists tauri, comrak and serde.
• cargo test --locked passes locally (9 tests).