feat(save): save-first E-01 to E-05 let logged-out learners save and sign in later (dev)

[navidshad]

The same change as #65 (into main), ported to dev. Logged-out learners can save a phrase. It is kept in the extension and sent to their account after they sign in. No server change.

Why: subturtle-doc docs/tech/save-first-implementation.md (rows save-first E-01 to E-05).

save-first E-01 to E-05 (same as #65)

• E-01: the selection popup button says "Learn more or Save" ("Learn" when saved, also for a phrase saved on this browser). The help text matches.
• E-02: a logged-out learner gets two buttons on the word card from the first phrase: "Save without login" and "Login to save" (which keeps save-login-prompt_viewed / _clicked; the new button sends save-without-login_clicked). The phrase goes to chrome.storage.local (pendingSaves), one copy per phrase. phrase_saved gets logged_in: false (and true for signed-in saves).
• E-03: from the 3rd local phrase, a "Sign in to keep your N phrases and review them tomorrow" card with Sign in / Not now. At 50, it replaces Save. Events save-signin-card_viewed / _clicked / _dismissed.
• E-04: after sign-in, each local phrase goes to the account with createPhrase and deviceId, and leaves the list only when the server accepts it. One run at a time; it stops at the save limit and keeps the rest. Event pending-saves_synced.
• E-05: the toolbar popup shows "N phrases waiting. Sign in to keep them".

See #65 for the details.

What is different on dev: languages

• The save window's language row is unchanged. It is still there for signed-in learners.
• Logged out there is no language state. So a local save also keeps the detected phrase language.
• During the sync, each phrase gets its language with the save window's own rule (resolveSaveRow):
  • a language the learner has: saved there;
  • a language not added yet, or an unknown one: saved to the active language (like "Save to ").
• The language goes with the bundle suggestion, the bundle (find and create) and createPhrase. One page saved in two languages gets a bundle per language.
• Without language profiles (dark launch), nothing is sent, the same as before.
• If the login has not loaded the language state yet, the sync loads it first.

Decisions different from the work list

• Bundle name: chosen at sync time from the saved page URL and title (the suggestion needs a signed-in user). The fallback bundle is "Saved phrases". Same as feat(save): save-first E-01 to E-05 let logged-out learners save and sign in later #65.
• Language not added yet: the sync saves it to the active language. It does not keep it pending (keep-phrase-pending), because the learner is not there to choose "Add now".

How tested

• The tests from feat(save): save-first E-01 to E-05 let logged-out learners save and sign in later #65, plus three language cases in pending-saves-sync.test.ts (target language per phrase, bundle in that language, one bundle per language for one page).
• yarn typecheck: clean.
• yarn test: 49 files, 437 tests pass.
• yarn build: OK.
• yarn test:e2e (CI stub settings): 22 of 22 pass.
• The repo has no lint script.
• Not checked by hand in Chrome yet.

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