fix(ui-lvgl): 16 KB UI stack (screenshots reset the device); USB popup after the splash

The UI runs on Arduino's loop task, whose stock 8 KB stack had 340 B left
at its deepest point; a screenshot's file write went past it and reset the
device. The loop stack is now 16 KB (measured peak ~7.8 KB) and the
screenshot's line buffer is static. Diagnostics names the row
"UI stack free (lowest)".

With a cable in at boot, the USB popup waited for nothing and showed over
the splash; it now comes once the splash has faded.

Adds docs/development/plan-next.md, the plan up to the merge into dev.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/development/plan-next.md

@@ -0,0 +1,109 @@
+# Plan: from here to the merge into dev (2026-09-28)
+
+Work in order; each item is implemented, checked in the sim, flashed, and
+committed only after it's accepted. Tick items off as they land. Working
+file: goes in the repo clean-up (stage I).
+
+## A. Bugs
+
+- [x] **A1. Restart on a screenshot + "340 B stack free".** Likely one cause.
+      Diagnostics shows the UI loop task's lowest-ever stack headroom
+      (`uxTaskGetStackHighWaterMark`); the Arduino loop stack is 8 KB, so
+      340 B left is nearly an overflow. `takeScreenshot()` keeps a 960 B line
+      buffer on that stack, plus the FAT write.
+      1. Confirm with Diagnostics > Last crash after a screenshot restart.
+      2. Move the line buffer off the stack.
+      3. Raise the loop stack (internal RAM).
+      4. Measure the headroom on heavy screens (map, history, OTA).
+      5. Label the Diagnostics row with the task it measures.
+      Done 2026-09-28: loop stack 16 KB (SET_LOOP_TASK_STACK_SIZE), line
+      buffer static, row "UI stack free (lowest)". Measured 8532 B free
+      after map, history and screenshots, so the UI peaks at ~7.8 KB --
+      the old 8 KB left 340 B. Could go down to 12 KB if RAM gets tight.
+- [x] **A2. USB popup before the splash screen is gone.** `usbPoll()` waits
+      for the splash to finish; a cable plugged in at boot shows the popup
+      right after it.
+
+## B. Map
+
+- [x] **B1. Map menu order.** List the current entries, propose groups and
+      an order (view, overlays, areas & downloads, trails, test), agree on
+      it before coding. Done: TRAIL / LIVE SHARE / ARRIVAL ALERT (each with
+      its own options page, one section of PG_NAV), OFFLINE MAPS (Map areas,
+      Live tiles; "Download an area" dropped, a running / unfinished
+      download shows at the top of Map areas), LAYERS.
+- [x] **B2. Downloaded areas.** Selecting an area previews it at once (no
+      Show button); leaving the menu hides it and restores the previous map
+      view. Fixes the area staying on the map after Show.
+
+## C. Diagnostics: the noise tests (to think over)
+
+Diagnostics stays as it is. Only the Noise tab is in question: the tests that
+found the interference source (noise floor with the board's parts off one by
+one, the 850-930 MHz and mesh-channel sweeps, the spike hunt, the 15 s states
+for a second radio). They are L2-specific and block the loop for ~90 s.
+
+- [ ] Decide: remove them, or keep one universal tool (for example a noise
+      floor sweep round the mesh frequency that works on any board) and drop
+      the L2 part toggling and spike hunt.
+
+## D. Quiet hours in the core
+
+- [ ] A start and end time when the device is silent; in ui-core, so L1, L2
+      and other boards get it. New NodePrefs fields go at the end of the
+      stored layout, so an L1 keeps its settings after the update.
+      To decide: the Clock alarm rings anyway (proposed yes); whether a
+      message wakes the screen during quiet hours; with the clock not set,
+      quiet hours are off (proposed).
+
+## E. OTA end-to-end test
+
+- [ ] After the feature changes: a real release with an L2 asset (the v3 env
+      must be the `*_solo_lvgl` one before tagging), install over WiFi.
+      Publishing the release needs confirmation first.
+
+## F. The big review
+
+- [ ] Dead code; similar elements written several times, merged into one;
+      places to speed things up or save RAM, flash and battery.
+      First a list of findings to accept, then fixes in batches, each checked
+      in the sim and on the device.
+
+## G. Core parity with L1 SOLO
+
+- [ ] A feature table: L1 SOLO / ui-core / L2 (ui-lvgl). What's missing goes
+      into ui-core, not into each UI separately.
+
+## H. Before the merge into dev
+
+- [ ] The pull request about the lock screen.
+- [ ] The new issues.
+- [ ] Merge into dev.
+- [ ] A last review.
+
+## I. Repo, documentation, website, firmware tiers (its own detailed plan)
+
+- [ ] Repo: remove working files (plans etc.), keep only the project's code;
+      README down to the minimum; add Buy Me a Coffee.
+- [ ] Documentation written from scratch: short, describing the firmware's
+      features, easy to browse; sections marking where devices differ; no
+      screenshots of every screen and device. It stays in the repo, its
+      official entry is the simulator website.
+- [ ] Website rebuilt and polished; the simulator lets you pick which
+      firmware to simulate.
+- [ ] Firmware in three tiers, one shared core, extras (map tiles etc.) by
+      what the hardware can do, making full use of each device:
+      - minimal -- nRF52, a limited UI, small e-ink and OLED;
+      - standard -- mostly ESP32, large e-ink and LCD, higher resolutions,
+        a somewhat richer UI;
+      - color -- colour touch screens.
+
+Proposed: write a short tier spec (which boards, which features, what's in
+the core) as a document before stage F, so the review consolidates towards
+it; implement the tiers, docs and website after the merge.
+
+## Open decisions
+
+- C: remove the noise tests or keep a universal version.
+- D: alarm during quiet hours; messages waking the screen.
+- I: tier spec before stage F, or after the merge.