feat: long-press Confirm launches KOReader sync from EPUB (#1808)

## Summary
- Hold the menu (Confirm) button for ≥1s while reading an EPUB to launch
the existing `KOReaderSyncActivity` directly — replaces the three-step
path (open reader menu → scroll to Sync → confirm) with a single
gesture.
- Reuses `ReaderUtils::GO_HOME_MS` (same 1s threshold used by long-press
Back) and the existing `KOREADER_STORE.hasCredentials()` guard.
- Adds a Controls picker **"Long-press Menu"** (`longPressMenuFunction`,
default **Bookmark**) that **cycles through the available functions**
bound to the long-press gesture: `KOSync → Disabled → Bookmark`. The
field name and `LONG_PRESS_MENU_FUNCTION` enum are intentionally general
so future actions (dictionary lookup, table of contents, etc.) can be
appended without another schema migration. The setting is **not** a
binary toggle.
- Existing menu Sync entry still works — both call sites share one
extracted helper (`launchKOReaderSync`); no logic duplication.
- Short-press Confirm release is gated on duration so the reader menu
does not also open after a long press that *acts*, mirroring the
existing long-press Back pattern.
- **No-credentials fall-through:** `launchKOReaderSync()` now returns
whether it acted. When the function is set to KOSync but no KOReader
credentials are stored, the long-press is a no-op that **falls through
to open the reader menu** — so the menu stays reachable instead of the
hold silently swallowing the gesture. The release is only suppressed
when sync actually launched or surfaced a save error.

## Test plan
- [x] `pio run` succeeds clean for the `default` ESP32-C3 environment.
- [x] On-device, value **KOSync**, valid KOReader credentials:
long-press Confirm ≥1s → sync screen launches; get + update progress
return HTTP 200; release does **not** also open the reader menu; returns
to the same page.
- [x] On-device, value **KOSync**, **no** credentials: long-press
Confirm falls through and **opens the reader menu** (regression fix);
short-press also opens the menu.
- [x] On-device: menu → Sync still launches the same screen (shared
helper) and syncs (200/200).
- [x] On-device, value **Bookmark**: long-press drops a bookmark and
does **not** also open the menu.
- [x] On-device, value **Disabled**: long-press Confirm opens the menu
on release; no sync, no bookmark.
- [x] On-device: long-press Back still goes to the file browser
(unchanged path).
- [x] Heap: epub is released before the TLS handshake (frees ~16 KB);
min free heap stayed ~84 KB during sync, well above the safe floor. No
panics/OOM across the session.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

USER_GUIDE.md

@@ -241,6 +241,10 @@ The Settings screen allows you to configure the device's behavior. There are a f
   
   - "Chapter Skip" (default) - Long-pressing skips to next/previous chapter
   - "Page Scroll" - Long-pressing scrolls a page up/down
+- **Long-press Menu**: Selects the function bound to holding the menu button (Confirm) while reading an EPUB. **Cycles through the available functions** each time the setting is selected — additional functions may be added in future releases, so this is not a binary on/off toggle. A short press of Confirm always opens the reader menu as normal:
+  - "Bookmark" (default) - Hold Confirm (~0.4 second) to drop a bookmark at the current page.
+  - "KOSync" - Hold Confirm (~1 second) to launch KOReader sync directly.
+  - "Disabled" - Long-press is ignored; only short-press opens the reader menu.
 
 - **Short Power Button Click**: Controls the effect of a short click of the power button:
   
@@ -528,6 +532,7 @@ When reading an EPUB that contains footnotes, you can navigate to the footnote t
 * **Return to Home:** Press the **Back** button to close the book and return to the **[Home](#31-home-screen)** screen.
 * **Return to Browse Files:** Press and hold the **Back** button to close the book and return to the **[Browse Files](#33-browse-files-screen)** screen.
 * **Reader Menu:** Press **Confirm** to open the **[Reader Menu](#5-reader-menu)**, which includes chapter navigation, reading options, and more.
+* **Long-press Confirm (configurable):** Holding **Confirm** runs the function chosen by the **Long-press Menu** setting in **[Controls Settings](#363-controls)** — "Bookmark" (default) drops a bookmark, "KOSync" launches KOReader Sync, "Disabled" does nothing. A short press always opens the Reader Menu.
 
 ### Supported Languages
 
@@ -574,9 +579,9 @@ Accessible by selecting **Chapters** from the Reader Menu.
 
 Bookmarks can be created to quickly save and restore your place in a book.
 
-To create a bookmark, hold **Confirm** for 1 second while inside a book. A popup will appear letting you know a bookmark was created. The popup message will automatically disappear in a couple of seconds.
+To create a bookmark, hold **Confirm** for about half a second while inside a book. A popup will appear letting you know a bookmark was created. The popup message will automatically disappear in a couple of seconds.
 
-To open bookmarks, press **Confirm** while inside a book. Then navigate to the **Bookmarks** menu. Bookmarks can be opened by navigating to them and pressing **Confirm**, which will redirect you to that place in the book. You can delete bookmarks by holding **Confirm** for 1 second, and then pressing **Confirm** again to confirm deletion, or **Back** to cancel.
+To open bookmarks, press **Confirm** while inside a book. Then navigate to the **Bookmarks** menu. Bookmarks can be opened by navigating to them and pressing **Confirm**, which will redirect you to that place in the book. You can delete bookmarks by holding **Confirm** for about 0.7 seconds, and then pressing **Confirm** again to confirm deletion, or **Back** to cancel.
 
 Bookmarks are stored in the `.crosspoint/bookmarks` folder in the JSON format.