feat(heltec): solo dual-transport builds for Heltec V3/V4

Port the Wio Tracker L1 solo firmware (full on-device UI, dual BLE/USB
companion transport) to Heltec V3 and V4 OLED boards. Neither board has
a joystick or CardKB on-board, so each new env wires up both as optional
peripherals with default pins from what the board leaves free, gated
behind the existing UI_HAS_JOYSTICK/ENV_PIN_SDA+SCL flags.

DUAL_SERIAL was nRF52-only; added an ESP32 helpers/esp32/DualSerialInterface.h
counterpart so the flag isn't silently ignored on these boards. On V4's
native USB CDC, isClientConnected() also honours (bool)Serial (real DTR),
same as the nRF52 version; V3 has no native CDC so it stays BLE-only there.

Screen (SDA 17/SCL 18) and CardKB (SDA 3/SCL 4) confirmed working on real
V4 hardware.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

release-notes.md

@@ -8,6 +8,7 @@
 - **Bot Trigger fields accept multiple phrases.** Pack several trigger words into one Trigger field, comma-separated (`hi,hello there,yo`) — matching any one of them fires the reply, same as before for a single phrase.
 - **Remote Bot Actions gain `!gpio1`..`!gpio4`** (Wio Tracker L1 only — 4 otherwise-unused pins). Each pin is independently set to Off/Input/Output (GPIO1/GPIO2 also offer Analog) from a new **Tools › GPIO** screen; `!gpio1 on`/`!gpio1 off` drives an Output pin remotely, a bare `!gpio1` reports the current mode and reading (including a millivolt value in Analog mode). Gated by the same per-target Actions toggle as `!buzz`/`!gps`/`!advert`.
 - **Optional CardKB support (Wio Tracker L1, Grove I2C) — full keyboard-only navigation.** Plug an M5Stack CardKB into the Grove connector and it's auto-detected at boot — no setting to flip. Typing goes straight into the message/name field instead of navigating the on-screen keyboard grid, always as plain Latin text regardless of the Settings › Keyboard language/type — arrows and Esc work as expected everywhere else too (including inside the placeholder and accent popups). Enter acts like the physical centre button (advances the on-screen grid selection) rather than submitting, so **Fn+Enter** sends the message/confirms the field from anywhere, and **Fn+`<letter>`** opens that letter's accent popup directly (no arrow-hunting needed — handy for Polish/Czech/etc. diacritics — and works no matter what language/keyboard type is configured, since CardKB always types Latin either way). **Tab** is the Hold-Enter equivalent everywhere, including message reply/navigate and the Bot/Admin/Repeater menus — same single shortcut whether or not the on-screen keyboard is showing.
+- **Solo builds for Heltec V3 and V4** (`Heltec_v3_companion_solo_dual`, `heltec_v4_companion_solo_dual`) — the full standalone on-device UI instead of a screen that only mirrors the phone app, with dual transport so the companion app can attach over BLE or USB serial (BLE wins while both are live), same as the Wio Tracker L1 solo builds. Dual transport had only ever been built for nRF52, so ESP32 got its own `DualSerialInterface` to match. Both boards ship with a single button, which the solo UI can't be driven from, so each env enables both supported input devices with default pins picked from what that board leaves free: a **CardKB** on a second I2C bus (pair it with Ext. KB = Compact and no joystick is needed at all) and a **wired joystick** (four direction pins plus a Back button, wired straight to GND — the firmware enables the internal pull-ups). Either is enough on its own, and pins with nothing attached read as not-pressed, so the unused half costs nothing. The env comments list which GPIOs each board has already claimed, for anyone wiring theirs differently.
 - **GAT562 30S solo builds now use the same OLED font as the Wio Tracker L1.** The misc-fixed 6x9 font (full Latin, Greek and Cyrillic) had only ever reached the SH1106 and e-ink drivers, so on this board every keyboard alphabet beyond ASCII, and every accented contact name, was still being transliterated (`Łódź` → `Lodz`) or drawn as a filled block. Solo builds only — the font costs ~14 KB of flash and the repeater/companion builds have no keyboard to type those alphabets on; they keep the built-in font and are byte-for-byte unchanged.
 - **Settings › Keyboard gets an "Ext. KB" row** (boards that support CardKB, above). Switching it to **Compact** hides the on-screen letter grid, special-row icons, and status line entirely — a CardKB typist never looks at any of it, and none of it (script/page, T9-vs-ABC, caps) is actually actionable from CardKB anyway — replacing them with just a reminder of the Tab/Fn+letter shortcuts; the accent and placeholder popups still show normally. With the grid hidden, arrows and Tab take on more useful direct meanings instead of driving an invisible grid selection: **arrows** move the text cursor immediately (no more entering cursor mode first), **plain Enter** submits the message/field (same as Fn+Enter — there's no grid cell to commit), and **plain Tab** opens the placeholder picker directly. Compact is designed to need no joystick/physical button at all, and reclaims the freed space for extra message-preview lines. Off (**Full**) by default.