Open picked files through the content resolver, not std::fs

Loading a GPX on Android failed for every file in the picker. The dialog
plugin fires ACTION_GET_CONTENT, which returns a `content://` URI, and
`load_profile_from_path` handed that straight to `std::fs::read_to_string`
— "no such file or directory" for a file the rider is looking at. Picking
from Nextcloud makes it plainer: a document provider backed by a server
may have no local file at all until the resolver opens the stream, so
there was never a path to find.

So the command now takes a `FilePath` and reads through tauri-plugin-fs,
which opens a path directly and a URI via the resolver. The plugin is
here for `FsExt` alone; nothing in ui/ calls its commands, so the
capabilities are unchanged.

Two things that were derived from the filename can no longer be:

  - GPX is detected by content. A document id need not contain a name,
    let alone an extension. No YAML profile begins with `<`.
  - The route name falls back to the GPX's own <name>. Providers over
    real storage encode the filename in the last segment, but an opaque
    row id would have made a wretched route name.

save_fit had the same bug on the export side — PathBuf::from on a
save-dialog URI — and now writes down a resolver descriptor when handed
one.

Note for anyone rebuilding locally: gen/android/tauri.settings.gradle is
autogenerated and lists each plugin's Android project, so the new
plugin's Kotlin only reaches the APK after `cargo tauri android init` and
scripts/sync-android-sources.sh. CI already runs both.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src-tauri/Cargo.toml

@@ -22,6 +22,11 @@ bikecontrol-fit = { workspace = true }
 
 tauri = { version = "2", features = [] }
 tauri-plugin-dialog = "2"
+# Not for the frontend — nothing in `ui/` calls the fs commands. This is here
+# for `FsExt::read_to_string`, the one API that can read what the *picker*
+# returns on Android: a `content://` URI rather than a path. See
+# `read_picked_file` in commands.rs.
+tauri-plugin-fs = "2"
 
 uuid = { workspace = true }
 chrono = { workspace = true }