fix(core,app,android,cli,packaging): harden account sign-in and watch-history sync

- Desktop shows Proton's CAPTCHA instead of failing the sign-in: a wry webview
  in a child process of the app (`pstr-app::verify`), confined to
  https://*.proton.me, hands the token back and the sign-in is retried. Linux
  packages and CI gain WebKitGTK 4.1; the Windows zip and MSI ship
  WebView2Loader.dll, without which a -gnu build does not start.
- The desktop keyring splits a secret too long for one entry across several,
  so a Proton session with many key salts survives Windows' 1280-unit cap.
  Short secrets stay one entry, so every existing secret still reads.
- `WatchSync` runs against a small `HistoryDrive` trait and is tested end to
  end against an in-memory Drive: two devices, newer wins, no re-upload of an
  unchanged history, a missing file written again, an unreadable own file
  never overwritten, a newer-format file left alone.
- An installation's file unwritten for 90 days is retired by the next device
  to sync, after its entries are merged and carried in that device's file.
- Removing a share keeps its watch state: sync would bring it back from the
  other devices anyway, and re-adding the share picks up where it left off.
- Watch history syncs once more on desktop exit (bounded to 3 s) and, on
  Android, through a WorkManager job enqueued when the app leaves the screen.
- The second-factor prompts say security keys are not supported.

crates/pstr-cli/src/main.rs

@@ -1084,7 +1084,6 @@ async fn play(
     Ok(())
 }
 
-/// Unix seconds, for the "when was this asked" columns.
 /// Walk an interactive sign-in through whatever the account asks for.
 async fn login(accounts: &AccountStore, username: &str) -> Result<()> {
     let password = read_password("Password: ")?;
@@ -1092,10 +1091,12 @@ async fn login(accounts: &AccountStore, username: &str) -> Result<()> {
         Ok(step) => step,
         Err(pstr_core::Error::HumanVerification(challenge)) => {
             // Solving a CAPTCHA needs a browser engine that can hand the
-            // token back, which a terminal does not have.
+            // token back, which a terminal does not have. The desktop app has
+            // one, and keeps its session in the same keyring entry.
             anyhow::bail!(
                 "Proton wants a CAPTCHA before it accepts this sign-in. Sign in once in the \
-                 desktop or Android app, or try again later. ({})",
+                 desktop app on this computer, which shows it and shares its session with \
+                 pstr, or try again later. ({})",
                 challenge.verification_url()
             );
         }
@@ -1108,7 +1109,8 @@ async fn login(accounts: &AccountStore, username: &str) -> Result<()> {
                 return Ok(());
             }
             SignIn::SecondFactor(pending) => {
-                let code = read_password("Two-factor code: ")?;
+                // Security keys need WebAuthn, which this sign-in does not speak.
+                let code = read_password("Code from your authenticator app: ")?;
                 pending
                     .second_factor(&code)
                     .await