fix: free Epub RAM and simplify KOSync navigation via ActivityManager (#1860)

## Summary

* **What is the goal of this PR?** Fix KOSync failing with "Network
error" on large/complex EPUBs, and simplify the sync navigation flow by
removing the callback/result pattern.

* **What changes are included?**

This PR combines the approaches from #1855 and #1760 into a single,
cleaner solution:

  **Memory fix (from #1855):**
- `EpubReaderActivity` pre-computes the local KOReader position and
chapter name, then explicitly releases `epub` and `section` before
launching `KOReaderSyncActivity`. This frees ~65KB measured on device,
giving the TLS handshake sufficient heap. The root cause was
`MBEDTLS_ERR_X509_ALLOC_FAILED` (-0x2880) when a 3-cert chain consumed
~48KB during the handshake with only ~50KB available.
- `KOReaderSyncActivity` no longer receives a `shared_ptr<Epub>` at
construction — it lazy-loads the Epub after TLS only if remote progress
is found (`ensureEpubLoaded()`).
- Added `MIN_HEAP_FOR_TLS = 55000` guard in `KOReaderSyncClient` —
returns `LOW_MEMORY` early if aggregate free heap is too low before
attempting a TLS connection.

  **Navigation simplification (from #1760):**
- Replaced `startActivityForResult` + callback with
`activityManager.replaceActivity` / `activityManager.goToReader`.
Progress is saved to `progress.bin` before the epub is released
(cancel/upload paths) and in `saveProgressAndReturn` (apply remote
path). The reader re-launches from the saved position naturally via
`goToReader`, eliminating the need to reload the epub in a callback.
- Extracted `ReaderUtils::saveProgress()` as a shared helper used by
both `EpubReaderActivity` and `KOReaderSyncActivity`.
- Added `STR_SAVE_PROGRESS_FAILED` to all 22 language files for the case
where writing the synced position to SD fails.

  **Orientation fix (found during device testing):**
- `EpubReaderActivity::onExit()` resets the renderer to portrait before
destruction. With `replaceActivity` the reader is fully torn down before
KOSync starts, so KOSync was always rendering in portrait even when
reading in landscape. Fixed by calling `ReaderUtils::applyOrientation`
in `KOReaderSyncActivity::onEnter()`.

## Additional Context

Heap measurements on device (large EPUB with complex CSS):

| Metric | Before | After |
|---|---|---|
| Heap before Epub release | 88,156 bytes | — |
| Heap after Epub release | — | 153,892 bytes (+65,736) |
| Heap at TLS handshake | ~50,000 bytes (fails) | ~116,384 bytes
(passes) |
| Min-free-ever during sync session | 2,600 bytes | 33,052 bytes |
| TLS result | `MBEDTLS_ERR_X509_ALLOC_FAILED` | HTTP 200 |

Tested on device: sync from inside a large EPUB in both portrait and
landscape, cancel, apply remote progress, upload local progress.

---

### AI Usage

Did you use AI tools to help write this code? _**YES**_

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/I18n/translations/russian.yaml

@@ -286,6 +286,7 @@ STR_NO_REMOTE_MSG: "Удалённый прогресс не найден"
 STR_UPLOAD_PROMPT: "Отправить текущую позицию?"
 STR_UPLOAD_SUCCESS: "Прогресс отправлен!"
 STR_SYNC_FAILED_MSG: "Ошибка синхронизации"
+STR_SAVE_PROGRESS_FAILED: "Не удалось сохранить прогресс"
 STR_SECTION_PREFIX: "Раздел "
 STR_UPLOAD: "Отправить"
 STR_BOOK_S_STYLE: "Стиль книги"